'shallow clone' the WordPress develop repo

There is no need to checkout the entire history of the WordPress develop Git repo.

Using `--depth=50` reduces the checkout time by ~75%

http://git-scm.com/docs/git-clone
> --depth <depth> Create a shallow clone with a history truncated to the specified number of revisions.

Time WordPress Git develop repo 'shallow clone' vs 'full clone'
https://gist.github.com/ntwb/36ad2a31cb87e7411c28

This resulted in 'my' build on Travis CI taking 22 seconds vs the previous Jetpack build taking 1min33secs
https://travis-ci.org/ntwb/jetpack/builds
https://travis-ci.org/Automattic/jetpack/builds
Shallow Clone One
Carousel: do_action documentation first pass
Comments: do_action docs added. First pass
Contact Forms: do_action docs. Initial pass
Contact Forms: Additional do_action docs
Custom CSS: do_action docs. Initial pass
Sharing: do_action docs first pass
Replace old photon tests with new ones.
Vimeo Autoplay: Modify how we're adding the arg

Only add the arg if it’s true, and use add_query_arg to add it
previously, so we’re not manually appending it.
Testimonial CPT: allow for pagination in testimonial archive. fixes #themes2626; fixes #premiumthemes633

Merges r103720-wpcom.
Likes: bump cachebuster to pick up new CSS (with fixed padding for RTL).

Merges r103766-wpcom.
Merge commit 'd90b463'
Omnisearch: remove pre-mp6 CSS

Props Jeremy Herve <jeremy@tagada.hu>
Authored on Sun Aug 10 15:05:03 2014 +0000
https://github.com/Automattic/jetpack/commit/002d259

***

This merges the following commits:
    - https://github.com/Automattic/jetpack/commit/002d259

Merges r103751-wpcom.
Merge commit 'eba45b8'
Switch how we detect autoplay.

This a bit more elegant and clear.
Vimeo: Switch the output iframe markup

Add new allowfullscreen html5 attributes that the Vimeo source now
includes.  Also sprintf, and make sure sure height and width are cast
as unsigned integers.
Vimeo: Add detection for loop arguments.
Vimeo: Add braces.  Code style. Trivial.
Merge branch 'master' of https://github.com/Automattic/jetpack