add pip-accel to travis to cache builds better

.travis.yml

@@ -1,12 +1,16 @@
 sudo: false
 language: python
-cache: pip
+cache:
+  pip: true
+  directories:
+    - ~/.pip-accel
 python:
 - '3.4'
 addons:
   postgresql: '9.3'
 install:
-- pip install -r requirements_for_test.txt
+- pip install pip-accel
+- pip-accel install -r requirements_for_test.txt
 before_script:
 - psql -c 'create database test_notification_api;' -U postgres
 script: