Manage AppEngine SDK version in Gradle tasks

[ggalmazor]

After PR #230, @yanokwa reported problems due to mismatched versions of the AppEngine SDK zip file that's stored in installer/appEngineSDK.zip.

The Gradle task will check for the zip file and download a fresh copy only if it's not found. When we change the SDK version, the user has to remove the zip file manually before using gaeUpdate or installerBuild Gradle tasks.

We could make the Gradle tasks a little bit more intelligent to check for SDK versions. Also, extracting the SDK version into a variable could help avoid repetition and errors throughout the code.