Fix: Use Bundler if Gemfile present, unless --skip-bundler specified.

This was accidentally introduced in ac024bf.

Now running RSpec for a project containing a Gemfile again uses Bundler, even if `--bundler` is not specified explicitly. Specify `--skip-bundler` to override.
Rearrange some lines to group methods on top.
Adapt TextMateFormatter for RSpec 3, use RSpec’s TextMateFormatter for RSpec 1 & 2.

RSpec 2 still ships its own TextMateFormatter, so it’s safe to make our formatter compatible with RSpec 3 only (taking care to not even require the file for older RSpec versions).
Only require text_mate_formatter if it’s really used.

A very small optimization, but nevertheless :-)

(Also makes further refactoring for using rspec binstub easier.)
Merge pull request #69 from die-antwort/make-compatible-with-rspec3

Adapt TextMateFormatter for RSpec 3, use RSpec’s own TextMateFormatter for RSpec 1 & 2