Cache pip downloads on Travis.

.travis.yml

@@ -4,6 +4,9 @@ python:
   - "3.3"
   - "3.4"
   - "3.5"
+cache:
+  directories:
+    $HOME/.cache/pip
 # install libjpeg-dev for Pillow to handle JPEGs
 sudo: false
 addons:

tox.ini

@@ -8,6 +8,7 @@ envlist = py27, py35
 
 [testenv]
 commands = py.test --cov=dosage --cov=dosagelib --tb=short -n10
+passenv = TRAVIS
 # Also install optional dependencies for tests.
 deps =
     pytest-xdist