Shebang snippet (rb⇥) works at begin-of-doc in non-ruby documents

Previously you could only use the tab trigger when the file type was set to Ruby (or something derived from Ruby), but if the caret is placed at the beginning of the document, then it will now work regardless of file type.

The chance of this conflicting with another tab trigger is pretty low, and a future version of TextMate may automatically switch file type when the first line is changed (to something matched by a grammars first-line-match), which is the motivation behind this change.