fix(jpegmini): the text field in sierra has changed to a combo box

closes #129

Debugging with UI Browser has shown me the input for JPEGmini has changed in
OSX 10.12.

The UI Browser report outputs:

```
application "JPEGmini"
    standard window "JPEGmini" (window 1)
        sheet (sheet 1)
            sheet (sheet 1)
                combo box (combo box 1)
```

Added OS X version check to maintain backward compatibility.

My first pass at logical changes were limited because EVAL doesn't seem to be
supported in Apple Script, could be improved with a better comparison for future
versions.

Updated to match code formatting and contribution guide

bin/imageOptimAppleScriptLib

@@ -111,14 +111,28 @@ on runJPEGmini()
   (* NAVIGATE TO OUR FOLDER OF IMAGES *)
   tell application "System Events"
     tell process JPEGMINI_APP_NAME
-      set value of text field 1 of sheet 1 of sheet 1 of window 1 to DIRECTORY_JPG
-      repeat
-        if (value of text field 1 of sheet 1 of sheet 1 of window 1) is not equal to DIRECTORY_JPG then
-          delay 1
-        else
-          exit repeat
-        end if
-      end repeat
+      (* < SIERRA, FILE PATH SELECTOR IS TEXT INPUT *)
+      if text field 1 of sheet 1 of sheet 1 of window 1 exists then
+        set value of text field 1 of sheet 1 of sheet 1 of window 1 to DIRECTORY_JPG
+        repeat
+          if (value of text field 1 of sheet 1 of sheet 1 of window 1) is not equal to DIRECTORY_JPG then
+            delay 1
+          else
+            exit repeat
+          end if
+        end repeat
+      end if
+      (* >= SIERRA, FILE PATH SELECTOR IS COMBO BOX *)
+      if combo box 1 of sheet 1 of sheet 1 of window 1 exists then
+        set value of combo box 1 of sheet 1 of sheet 1 of window 1 to DIRECTORY_JPG
+        repeat
+          if (value of combo box 1 of sheet 1 of sheet 1 of window 1) is not equal to DIRECTORY_JPG then
+            delay 1
+          else
+            exit repeat
+          end if
+        end repeat
+      end if
       -- give Finder time to resolve the path
       delay 2
       keystroke return

package.json

@@ -19,6 +19,7 @@
     "Martin Schürrer (https://github.com/MSch)",
     "Ramiro Araujol (https://github.com/ramiroaraujo)",
     "Simen Brekken (https://github.com/sbrekken)",
+    "Stefan Crain (https://github.com/stefancrain)",
     "Tom Chenl (https://github.com/tomchentw)"
   ],
   "devDependencies": {

src/imageOptimAppleScriptLib

@@ -111,14 +111,28 @@ on runJPEGmini()
   (* NAVIGATE TO OUR FOLDER OF IMAGES *)
   tell application "System Events"
     tell process JPEGMINI_APP_NAME
-      set value of text field 1 of sheet 1 of sheet 1 of window 1 to DIRECTORY_JPG
-      repeat
-        if (value of text field 1 of sheet 1 of sheet 1 of window 1) is not equal to DIRECTORY_JPG then
-          delay 1
-        else
-          exit repeat
-        end if
-      end repeat
+      (* < SIERRA, FILE PATH SELECTOR IS TEXT INPUT *)
+      if text field 1 of sheet 1 of sheet 1 of window 1 exists then
+        set value of text field 1 of sheet 1 of sheet 1 of window 1 to DIRECTORY_JPG
+        repeat
+          if (value of text field 1 of sheet 1 of sheet 1 of window 1) is not equal to DIRECTORY_JPG then
+            delay 1
+          else
+            exit repeat
+          end if
+        end repeat
+      end if
+      (* >= SIERRA, FILE PATH SELECTOR IS COMBO BOX *)
+      if combo box 1 of sheet 1 of sheet 1 of window 1 exists then
+        set value of combo box 1 of sheet 1 of sheet 1 of window 1 to DIRECTORY_JPG
+        repeat
+          if (value of combo box 1 of sheet 1 of sheet 1 of window 1) is not equal to DIRECTORY_JPG then
+            delay 1
+          else
+            exit repeat
+          end if
+        end repeat
+      end if
       -- give Finder time to resolve the path
       delay 2
       keystroke return