Use Redis 3 for tests

We want to be able to run our tests using Redis 3, but we need to
manually install it via the APT addon.

.travis.yml

@@ -4,8 +4,10 @@ sudo: false
 
 cache: bundler
 
-services:
-  - redis-server
+addons:
+  apt:
+    packages:
+      - redis-server
 
 rvm:
   - 1.9.3
@@ -20,6 +22,7 @@ before_script:
   - git config --local user.name "Travis CI"
 
 script:
+  - redis-cli --version
   - bundle exec rspec
   - bundle exec overcommit --sign
   - bundle exec overcommit --run