Fixate test sample repos

Checks out a stable branch of the sample repos we use for acceptance testing. 

Notes: 
* this kicks in only when performing a release version of bill's kitchen (i.e. version number in Rakefile without the `-SNAPSHOT` suffix)
* for non releases the master branch of the sample repos is used

Also cleans up the outdated (btw already commented out) samples from the Rakefile as any new code samples should be embedded into the acceptance tests.