Make Travis CI limit the Git clone depth even more

In order to speed up the tests a little bit more, this commit informs
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/server-configs-apache.git
 ...

 real    0m2.549s
 user    0m0.068s
 sys     0m0.048s

 $ time g clone --depth 50 git@github.com:h5bp/server-configs-apache.git
 ...

 real    0m3.139s
 user    0m0.128s
 sys     0m0.024s

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

Ref: http://git-scm.com/docs/git-clone
     https://twitter.com/travisci/status/288390896339267584