Take advantage of the travis-ci cache feature

.travis.yml

@@ -5,14 +5,18 @@ env: # These should match the tox env list
     - TOXENV=py33
     - TOXENV=py34
     - TOXENV=pypy
-install: pip install coveralls tox --use-mirrors
+install: pip install coveralls tox
 script: tox
 # Special snowflake.  Our tests depend on making real commits.
 before_install:
-  - git config --global user.name "Travis CI"
-  - git config --global user.email "user@example.com"
-  # Our tests inspect some of *our* git history
-  - git fetch --unshallow
+    - git config --global user.name "Travis CI"
+    - git config --global user.email "user@example.com"
+    # Our tests inspect some of *our* git history
+    - git fetch --unshallow
 after_success:
     - coveralls
 sudo: false
+cache:
+    directories:
+        - $HOME/.cache/pip
+        - $HOME/.pre-commit