travis: cleanup and bump go versions

* cleanup structure
* explicitly use go-{1.5.3,1.6.2}

.travis.yml

@@ -2,20 +2,20 @@ language: go
 sudo: required
 dist: trusty
 
-go:
-  - 1.5.3
-  - 1.6
-  - tip
 
 matrix:
-  allow_failures:
+  include:
+    - go: 1.5.3
+      env: GO15VENDOREXPERIMENT=1
+    - go: 1.6.2
+    - go: tip
+  allow_failures: 
     - go: tip
 
 env:
   global:
     - TOOLS_CMD=golang.org/x/tools/cmd
     - PATH=$GOROOT/bin:$PATH
-    - GO15VENDOREXPERIMENT=1
 
 install:
  - go get ${TOOLS_CMD}/cover