Suppress CMake unused variable warning for Android platform.
For Travis CI - simplify the ci_rebase rake task.
On normal commit, use the origin/master. On git tagging commit, use the 'tag' branch.
Also when on git taggig commit, prevent API doc update on the master branch because it is already too late to do so as the commit won't be in the tag anyway.
For Travis CI - switch CI build to use Android build environment.