Allow Travis to install requirements.txt

[jayvdb]

The pip --use-mirrors option was deprecated in version 1.5, and removed in 7.0, and 7.1.2 is now included with Travis' Python 3.5.0 environment.

Travis automatically installs packages in requirements.txt if stage install does not exist, and can cache the packages.

---

This change is 

[jayvdb]

The commit message doesnt make it extremely clear : the builds are currently red because if Python 3.5 as can be seen at https://travis-ci.org/jayvdb/nose/jobs/126686336