Update .travis.yml to build Go 1.6

.travis.yml

@@ -1,14 +1,20 @@
 language: go
 sudo: false
-go:
-  - 1.3
-  - 1.4
-  - 1.5
-  - tip
+
+matrix:
+  include:
+    - go: 1.2
+    - go: 1.3
+    - go: 1.4
+    - go: 1.5
+    - go: 1.6
+    - go: tip
+
 install:
   - go get golang.org/x/tools/cmd/vet
+
 script:
   - go get -t -v ./...
-  - diff -u <(echo -n) <(gofmt -d -s .)
+  - diff -u <(echo -n) <(gofmt -d .)
   - go tool vet .
   - go test -v -race ./...