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/html5-boilerplate.git
  ...

  real    0m2.636s
  user    0m0.052s
  sys     0m0.040s

  $ time g clone --depth 50 git@github.com:h5bp/html5-boilerplate.git
  ...

  real    0m3.542s
  user    0m0.064s
  sys     0m0.072s

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

Ref: http://git-scm.com/docs/git-clone
     https://twitter.com/travisci/status/288390896339267584
Update the content from the `dist` directory