Fast-finish Travis builds

The PHP7 build includes code coverage, which makes it run a bit longer now.
In general, we shouldn't wait for allowed_failures with build feedback.
Travis seems to run builds (roughly) in order, so allowed_failures
tend to prolong build times, since it only runs a few builds concurrently.

We'll move the PHP7 build out of allowed_failures eventually,
but I think the general concept still applies:
There's little value in waiting around for allowed_failures builds.

See https://blog.travis-ci.com/2013-11-27-fast-finishing-builds/

.travis.yml

@@ -16,6 +16,7 @@ env:
     - secure: "SDGv49c2Ee2YBz7dATE3WnHSVSvJiRJ2BVtRasVshdNDNz3NBRzh13C2fDwTGBU1J6PxiQaGTXBy/BGsvbYk2BvdzHVwozkBpHVSaCNdarpCJ5yZZTqKC3mpA1S5353r5tqronwFuMDpftzXnRMfLZGGQ4kYb9hjV55+FPUTFPk=" # Encrypted ARTIFACTS_SECRET
 
 matrix:
+  fast_finish: true
   include:
     - php: 5.5
       env: DB=MYSQL PHPUNIT_TEST=1