Force com.android.support:appcompat-v7 dependency to 20.0.0

The newest appcompat (21.x) does not support uses-sdk:minSdkVersion
less than 14.

For developers having the latest appcompat available, the build will
fail with + as the version number for com.android.support:appcompat-v7.
Merge pull request #4 from curinir/fix-appcompat-dependency

Force com.android.support:appcompat-v7 dependency to 20.0.0