Move Travis CI checkout and patching to `before_install` section.

Review Request #10976 — Created March 27, 2020 and submitted — Latest diff uploaded


In repos where dependencies are changing between branches, having the
checkout and patch steps run as part of the Travis CI script would
sometimes fail (for example, attempting to do builds of djblets on
release-1.0.x failed in install because it was trying to get
django-evolution 2.0, as required in the master branch).

Doing the checkout and patch in before_install makes a lot more sense,
because then we've got the full change in the working directory before
doing anything.

Ran a build of a change from djblets-1.0.x and saw that it got past the
initial install/setup steps, where before it didn't.