Update qhull to 2015.2

[ianthomas23]

This PR updates the qhull source code that we ship with matplotlib to be the latest version, 2015.2. There is no change in functionality.

I was going to change it to use the new reentrant version of qhull which is somewhat more thread safe. However, in trying this I unearthed a bug in debian-based distributions which ship the reentrant header files but not the shared library (libqhull_r.so). I have reported this to debian many weeks ago (https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=841168), but there hasn't even been an acknowledgement of the bug yet never mind a fix. Shipping mpl using the reentrant version whilst this bug still exists would result in lots of failed installs, so I have stuck to the original non-reentrant version and will write another PR when debian fix the bug.

Assuming this passes the tests and everyone is happy with it, it will close issue #6791.

[tacaswell]

Restarted the 2 failed tests (look transient to me).

I see no reason not to do this.

[tacaswell]

@ianthomas23 Can you add a note to whats_new or api_changes (which ever you think is better) noting we bumped the vendored version.

[codecov-io]

Current coverage is 62.12% (diff: 100%)

Merging #7747 into master will not change coverage

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[ianthomas23]

@tacaswell I've added an entry to api_changes, and squashed that commit.

[NelleV]

Thanks @ianthomas23 !