Use non-sudo apt in travis

.travis.yml

@@ -1,5 +1,10 @@
 language: python
-sudo: required
+
+addons:
+  apt:
+    packages:
+    - redis-server
+
 python:
   - "2.6"
   - "2.7"
@@ -10,8 +15,7 @@ python:
   - "3.5-dev" # 3.5 development branch
   - "nightly" # currently points to 3.6-dev
 # command to install dependencies
-before_install:
-  - sudo apt-get install redis-server
+
 install: 
   - "pip install -r requirements.txt"     
   - "pip install -e ."