Bump version to 1.15.0

Changes since 1.14.0:
  * Implement maktaba#buffer#GetVisualSelection helper (#132).
  * Implement maktaba#log#GetFormattedEntries and support 1-arg log
    handler (see #204).
  * Notify user for messages logged at WARN or above by default (#185).
  * Fix Syscall.CallAsync failing for complex commands (#205).
  * Preserve trailing slash in maktaba#path#Split and MakeRelative
    (#137 and #175).
  * Fix 'maxfuncdepth' error in maktaba#json#Parse corner case (#153).

addon-info.json

@@ -1,7 +1,7 @@
 {
   "name": "maktaba",
   "description": "Consistent Vimscript",
-  "version": "1.14.0",
+  "version": "1.15.0",
   "author": "Google",
   "repository": {"type": "git", "url": "git://github.com/google/maktaba"}
 }