travis: set pip mirrors in ENV

Tiny patch for CI settings that I came across.
Setting mirrors for pip in env should also propagate to tox (making pip in tox use mirrors).
But if it doesn't, those additional settings to tox.ini should make it so:
```patch
 [tox]
 envlist = py27, pypy, py33
+setenv =
+    PIP_USE_MIRRORS=true
 
 [testenv]
```