Enable building Scala 2.9.2 applications for Android using OpenJDK 1.7

Fedora 17+ has OpenJDK 1.7. Building a Scala App using sbt, produces error messages like the one below:

[error] (My_Android_Project/android:proguard) java.io.IOException: Can't read [/home/shailesh/projects/android/my_android_project/target/scala-2.9.2/classes] (Can't process class [my/android/project/R.class] (Unsupported version number [51.0] for class format))

See https://github.com/jberkel/android-plugin/issues/102 for a similar error.

So, we just target JDK 1.6, and things work merrily.
Merge pull request #8 from shaileshkhandekar/master

Enable using JDK 1.7