Merge pull request matplotlib#5741 from tacaswell/tst_cache_on_travis

TST: enable caching on travis

Author: jenshnielsen

.travis.yml

@@ -1,5 +1,10 @@
 sudo: false
 
+cache:
+  directories:
+    - $HOME/.ccache
+    - $HOME/.cache/pip
+
 addons:
   apt:
     packages:
@@ -55,12 +60,15 @@ before_install:
   # Install into our own pristine virtualenv
   - virtualenv --python=python venv
   - source venv/bin/activate
+  - export PATH=/usr/lib/ccache:$PATH
 
 install:
+  - ccache -s
   # Upgrade pip and setuptools. Mock has issues with the default version of
   # setuptools
   - |
     pip install --upgrade pip
+    pip install --upgrade wheel
     pip install --upgrade setuptools
   # Install only from travis wheelhouse
   - if [ -z "$PRE" ]; then