Update dependencies and fix exception thrown by GSON

# Updated Dependencies
 * Retrofit updated to `1.7.1`
 * OkHttp updated to  `2.1.0-RC1`

As a result of the updates, a few members needed to be updated due to not being included in the new versions. 
 *  `HttpResponseCache` is now `Cache` 
 *  `RestAdapter.Builder#setServer()` is now `RestAdapter.Builder#setEndpoint()`

# Updated Gradle
Updated to version `0.13.2`. Updated `build.gradle` files to accommodate the new plugin names.

# Fix GSON IllegalStateException
GSON failed to parse the response data from a call to `ComicManager#getStoriesForComicId(...)`. It appears `connectionURL` is now just a `String`.