Accept branches containing slash '/' character

Created by: timosmit

I have several development branches (dev/1.1.0, dev/1.2.0, etc) which I would like to add to my tracking as well. The thing is, when adding these, the plugin aborts with the error that there is an invalid character in my branch list (supposedly the '/' char).

I'm currently testing a small patch of source-integration/SourceGithub/SourceGithub.php in which the regex on line 211 accepts the slash character. Currently I have not experienced any issues with other parts of the plug-in; if this stays the same, I will submit a PR.

On a sidenote, escaping the dot '.' character in a character class should not be necessary (this relates to the same regex).