adding notice on top of sourceslist files
Useful to figure out what is managed by puppet and what isn't when
 not setting up the option to purge sourceslist files
Fixed regex to actually follow APT requirements

Previous implementation would overwrite the file name and
potentially include characters that would cause apt to ignore
the pref file
Block this from running on unsupported platforms.
Merge pull request #300 from apenney/fix-tests

Block this from running on unsupported platforms.
Merge pull request #298 from frconil/fix-apt-pin-regex

Fixed regex to follow APT requirements
Merge pull request #297 from frconil/sourceslist-comment

adding notice on top of sourceslist files