For Travis CI - Misc. minor changes to improve CI build.
- Remove other temporary workaround for MinGW CI build to see if changing to 'Release' build configuration is enough to fix the insufficient memory problem.
- Attempt to fix intermitten network problem when installing software packages for Rasperry-Pi CI build by delaying the massive sysroot git clone to the last. Read this blog (http://blog.travis-ci.com/2013-05-20-network-timeouts-build-retries/) on general network issue plaguing travis-ci.org.
For Travis CI - Refactor to terminate early on any errors.
The errors include: git clone error, apt-get update/install error, download error, etc.
The purpose of this refactor is to clearly distinguish between Travis CI build error (caused by error on Travis side) and Travis CI build failure (caused by build or test failure on Urho3D side). Previous setup sometimes allowed build environment in error state to continue to proceed with software building phase due to inefficient branching logic.