Enable pip caching

Bug: T106070
Change-Id: I5a14d3e124cdb2833760dd2eaded396c8ba80550
---
M .travis.yml
1 file changed, 2 insertions(+), 0 deletions(-)

diff --git a/.travis.yml b/.travis.yml
index 4108055..70efc91 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -21,6 +21,8 @@
       - graphviz
       - liblua5.1-0-dev
 
+cache: pip
+
 before_install:
   # When PYSETUP_TEST_EXTRAS is not enabled, do not allow the
   # default 'install' step to install all dependencies listed in
