Fix exists? and unset for groups, fix test running.

[jfharden]

This PR fixes test running, adds a couple of tests for parsing.

Also fixes exists? not reporting true when a group is set, and unset not fully unsetting a group.

All changes have tests added for them, and since they are only bugs I've only bumped the patch version.

Also allows erb to be loaded, but wont break existing yaml loading.

[jfharden]

The dependencies need locking down a lot more to be compatible with the old versions of ruby travis is building for, I don't know if I'll get time to work out which versions need locking too.