Start running railties builds first, since they take the longest finish

This should make the Travis CI build complete somewhat faster by having the first workers pick up the `railties` tests (which are the slowest) before others.

See comparison of https://travis-ci.org/rails/rails/builds/37816675 versus https://travis-ci.org/rails/rails/builds/37814459 to understand this change.

![screen_shot_2014-10-13_at_12_47_52](https://cloud.githubusercontent.com/assets/10308/4612954/cef82f06-52cf-11e4-8c80-0336c50dc014.png)

![screen_shot_2014-10-13_at_13_01_20](https://cloud.githubusercontent.com/assets/10308/4613033/e82a3090-52d0-11e4-9050-210ee5cf4cc8.png)