Misc. fixes in the build script to improve the release mechanism.
The following changes address the shortcomings of our current release mechanism detected from our last release.
- Define and set RELEASE_TAG environment variable only when a git tag conform to a release tagging matching regex "\d+\.\d+".
- Change the logic to allow the automated rake tasks to be triggered on a 'tag' branch properly.
- Fix the 'git rebase' command to use the branch pointed by TRAVIS_BRANCH environment variable as baseline, instead of assuming it to be always 'origin/master' as before.
- Change Urho3D library versioning CMake module to omit the version patch number when there is none, i.e. 1.32 instead of 1.32.0.
For Travis CI - switch CI build to use OS X build environment.