Allow Travis CI to use new infrastructure

Travis recently launched their new infrastructure on Amazon EC2 with improved resources. The only limitation is that they don't support sudo. http://blog.travis-ci.com/2014-12-17-faster-builds-with-container-based-infrastructure/
I couldn't find anywhere where the build system uses sudo so I don't think that limitation affects rust builds. 
This change drops the current `make tidy` times [slightly](https://travis-ci.org/daramos/rust/builds).