chore(travis): cache pip in travis runs (#944)

Author: helgi

.travis.yml

@@ -9,6 +9,7 @@ services:
 sudo: required
 addons:
   postgresql: "9.4"
+cache: pip
 before_install:
   - wget "http://ftp.debian.org/debian/pool/main/s/shellcheck/shellcheck_0.3.7-5_amd64.deb"
   - sudo dpkg -i shellcheck_0.3.7-5_amd64.deb