Hardcode Go versions in .travis.yml
diff --git a/.travis.yml b/.travis.yml
index 8a6eebc..b56c60b 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,9 +1,9 @@
 language: go
 script: "./test.sh"
 go:
-    - 1.4.2
-    - 1.5.3
-    - 1.6
+    - 1.4.3
+    - 1.5.4
+    - 1.6.1
     - tip
 before_install:
   - go get github.com/axw/gocov/gocov