Update Travis CI configuration

- Include SDK v20 for Wear support.
- Switch to build tools v20.
Add debug keystore

A keystore needs to be present in order to enable continuous integration
builds. This is standard Android debug keystore with a 100 year
validity.
Update proguard config to exclude test classes

Reference to classes in test frameworks (e.g. JUnit) were causing
Proguard to fail while running the 'build' task.
Merge pull request #35 from tjohns/master

Update Travis CI configuration