core: enable fast_finishing (Travis CI)

fash_finish makes it so that if one job fails, the build is marked as
failed without having to wait for the other jobs to finish. If there are
still unfinished jobs at the time of failure, they continue running
after the build has been marked as failed.

.travis.yml

@@ -25,3 +25,5 @@ script:
 after_success:
     - weechat --help
     - weechat -r /quit
+matrix:
+    fast_finish: true