In Travis CI config, move environment setup to before_install, so as not to trigger failed builds

Environment setup on Travis CI can fail in a number of ways - failed git
checkouts, database connection problems, etc - that have nothing specifically
to do with BuddyPress's test configuration. This changeset moves those steps
into the before_install section, with the result that a failed setup routine
will result in an 'errored' rather than 'failed' build. Grunt setup and the
phpunit tests themselves will continue to generate failure messages when
something goes wrong.

See #5708

Props netweb

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8516 cdf35c40-ae34-48e0-9cc9-0c9da1808c22