travis: add pip cache

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>

.travis.yml

@@ -1,6 +1,9 @@
 language: python
 python: 2.7
 
+cache:
+  - pip
+
 install:
   - if [ "$HG_VERSION" != "dev" ];
     then pip install Mercurial${HG_VERSION+==$HG_VERSION};