Update minimum "extend" version (#12)

extend 3.0.1 has a known vulnerability, fixed in 3.0.2. Due to the way
that npm sometimes dedupes trees, the minimum version here appears to
occasionally pull in 3.0.1. Raise the minimum so that consumers don't
accidentally pull in a vulnerable dependency.

package.json

@@ -39,7 +39,7 @@
     "data-uri-to-buffer": "1",
     "ftp": "~0.3.10",
     "debug": "2",
-    "extend": "3",
+    "extend": "~3.0.2",
     "file-uri-to-path": "1",
     "readable-stream": "2"
   }