Use Travis CI built in pip cache support

https://docs.travis-ci.com/user/caching/#pip-cache
diff --git a/.travis.yml b/.travis.yml
index 237eccc..ff71948 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -2,9 +2,7 @@
 
 sudo: false
 
-cache:
-    directories:
-        - $HOME/.cache/pip
+cache: pip
 
 matrix:
   include: