travis: enable cache, should speed up builds a lot

This will allow travis builders to cache generated wheels between
builds, so all the dependencies don't have to get re-built every time.

.travis.yml

@@ -1,5 +1,8 @@
 sudo: false
 language: python
+cache: pip
+before_cache:
+  - rm -f $HOME/.cache/pip/log/debug.log
 python:
   - "2.7"
   - "pypy"