travis: cache pip
diff --git a/.travis.yml b/.travis.yml
index 9cd209b..9a9088f 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,5 +1,8 @@
 language: cpp
 sudo: false
+cache:
+  directories:
+  - $HOME/.cache/pip
 addons:
   apt:
     sources: