fix podspec dead link

Podspec format was changed to json and added ".json" postfix.
Therefore "spec" link has been dead link.

ref: https://github.com/CocoaPods/Specs/commit/b947340d648f51d66a6df1cbd1b5f7d8cf5a6de0
Merge pull request #39 from keqh/fix_podspec_link

fix podspec dead link