travis only 1.6 and 1.7

.travis.yml

diff --git a/.travis.yml b/.travis.yml
index 8ed1e44..9bc3fd0 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,8 +1,7 @@
 language: go
 go:
-  - 1.5
-  - 1.6.2
-  - stable
+  - 1.6
+  - 1.7
 
 install:
   - go get -v ./...