Update CONTRIBUTING guide.

A clean checkout won't run the specs without generating the
`lib/gitsh/version.rb` file first. Update the documentation to match.
Add travis.yml file.
Set up dependencies properly for Travis build.
Add code climate and travis badges to README.
Mention gh, which is faster than hub

Showing `hub` in the example will cause people to try it with `hub`,
which will lead to an unpleasant first experience due to `hub`'s speed.
By showing `gh` in the example, we hint at something better and help
popularize the faster alternative.

We see this in FactoryGirl, where people complain that FG is slow merely
because they write to the database often.
Change `current_branch_name` to use short branch name.
Emphasise use of ctrl-d in the README.

Unfortunately the example has to be re-formatted into HTML to mark this up
properly.
Update install instructions for 0.4.
Clarify the setup steps in the contributing guide.
Bug fix: Don't output greeting on ctrl-c

Catching an `Interrupt` and `retry`ing the method was re-running too many
things, including the greeting and the history load.
Support Ruby 1.9.3

This should significantly simplify installation on Linux, since 1.9.3 is
available as a package for most major distros. Also, it paves the way for us
building Debian packages (etc.) that only depend on standard packages.
Do not insert blank lines into `Readline`'s history