For Travis CI - code cleanup but add more memory diagnostics.
[ci package]

- Simplify the check on environment variable containing the commit-ish used in the 'git describe' command.
- Shuffle the rebase order in 'ci_rebase' rake task, move the OSX-CI rebase to the last as OSX Travis-CI build environment is extremely slow lately (has longer job queue).
- Improve 'android' rake task to only create named AVD for CI test when necessary and to turn off graphics acceleration explicitly when starting the emulator in CI environment.
- Add more memory diagnostics to help in investigation of process being killed by Travis CI upstream issue reported in #430.
For Travis CI - switch CI build to use Raspberry-Pi build environment.