Make Travis CI limit the Git clone depth even more

In order to speed up the tests a little bit more, inform Travis CI
to limit the Git clone depth to 10 commits (default is 50).

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

 e.g.:

    $ time g clone --depth 10 git@github.com:h5bp/html5boilerplate.com.git
    ...

    real    0m2.566s
    user    0m0.028s
    sys     0m0.032s

    $ time g clone --depth 50 git@github.com:h5bp/html5boilerplate.com.git
    ...

    real    0m3.660s
    user    0m0.048s
    sys     0m0.032s

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

Ref: http://git-scm.com/docs/git-clone
     https://twitter.com/travisci/status/288390896339267584
Use 2 space indentation in `.travis.yml`