add pip cache on travis

Davide Moro · Davide Moro · commit 317ef55aa3c0 · 2017-08-21T10:29:41.000+02:00
diff --git a/.travis.yml b/.travis.yml
@@ -1,4 +1,5 @@
 language: python
+cache: pip
 sudo: required
 python: 3.5
 services:
