Speed up CI builds by caching the pip directory (#1241)

* Speed up CI builds by caching the pip directory

* Add caching for Appveyor, too

* Make trivial change to appveyor.yml to restart build

* One more attempt at caching

Author: mdickinson

.travis.yml

@@ -3,6 +3,8 @@ dist: xenial
 services:
   - xvfb
 
+cache: pip
+
 addons:
   apt:
     packages:

appveyor.yml

@@ -1,5 +1,8 @@
 build: false
 
+cache:
+  - C:\Users\appveyor\AppData\Local\pip\Cache -> appveyor.yml
+
 environment:
   global:
     PYTHONUNBUFFERED: "1"