Travis: Use container-based infrastructure

http://blog.travis-ci.com/2014-12-17-faster-builds-with-container-based-infrastructure/

.travis.yml

diff --git a/.travis.yml b/.travis.yml
index 814710d..a4d6cc5 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,3 +1,4 @@
+sudo: false
 language: go
 go:
   - 1.1