For Travis CI - add memory diagnostics before attempting to git clone.
Temporary adjust build matrix to ignore RPI CI build error.
For Travis CI - misc. minor refactoring and bug fixes.
- Add Android API level environment variable.
- Prevent unnecessary enabling of CMake testing configuration when the build test cannot be performed for specific target platform.
- Fix missing 'abort' command on a number of error handling branches.
For Travis CI - switch CI build to use OS X build environment.