Tweak the travis file

- Build against more versions of Go
- Explicitely specify go versions

.travis.yml

@@ -1,14 +1,15 @@
 language: go
-matrix:
-  include:
-    - go: 1.4
-      env: TOOLS_CMD=golang.org/x/tools/cmd
+
+go:
+  - 1.4
+  - 1.5.3
+  - 1.6
+
+env:
+  - TOOLS_CMD=golang.org/x/tools/cmd
 
 install:
  - go get ${TOOLS_CMD}/vet
 
 script:
  - ./test
-
-notifications:
-  email: false