Bump Go versions in Travis config
diff --git a/.travis.yml b/.travis.yml
index 780c7ff..580ad22 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -3,8 +3,8 @@
 language: go
 
 go:
-        - 1.5
-        - 1.6
+        - 1.5.4
+        - 1.6.3
         - tip
 
 install: