For Travis CI - workaround RPI build intermittent build error.
Currently there is no permanent solution to the intermittent build environment setup issue. In order to avoid false alarm on the overall build status, this commit temporarily group the git clone step as part of the script run. As such when it exits non-cleanly, it would be treated as build failure instead of build error. But since we have already set the RPI CI build to allow failure, their 'failures' (if it happens) should not affect the overall build status.
For Travis CI - switch CI build to use OS X build environment.