Update .travis.yml to build Go 1.6

.travis.yml

@@ -1,8 +1,19 @@
 language: go
 sudo: false
 
-go:
-  - 1.3
-  - 1.4
-  - 1.5
-  - tip
+matrix:
+  include:
+    - 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 .)
+  - go tool vet .
+  - go test -v -race ./...