Merge pull request #289 from pyca/alex-patch-2

Enabling caching of pip downloads and wheels
diff --git a/.travis.yml b/.travis.yml
index 746a762..0e5be38 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,5 +1,10 @@
 sudo: false
 language: python
+
+cache:
+  directories:
+    - $HOME/.cache/pip
+
 env:
   global:
     - LC_ALL=en_US.UTF-8