Bump minimum travis version to 1.6

sfackler · sfackler · commit 86a9eb82a1ed · 2016-02-27T09:59:36.000-08:00
Required by serde_json
diff --git a/.travis.yml b/.travis.yml
@@ -3,7 +3,7 @@ sudo: required
 rust:
 - nightly
 - beta
-- 1.5.0
+- 1.6.0
 addons:
   postgresql: 9.4
 before_script:
