Enable RSpec in all ST versions

It's a known issue in the RSpec package that it's flagged as only available to ST2 users: https://github.com/SublimeText/RSpec/issues/34

I can confirm @jarhart's experience that it works fine in ST3. Nothing has happened on this for a while so I thought I'd submit a pull request and see if anyone complains.

I've just submitted a pull request to remove the RSpec TextMate bundle from Package Control: https://github.com/wbond/package_control_channel/pull/2321 – so if this is also merged it will mean ST3 users see the correct package in the install list.