Travis: Run gofmt -s, go vet, go test -race, add Go 1.6.

Also make tip a fast-finish allowed failure. That way, if CI fails on
tip due to a temporary issue with tip, it will not break build status.
However, it's still possible to see tip build status by looking at CI
details page.

Do not run go vet with Go 1.4 since it's not included in the standard
library, and it's no longer available in external standard library.

.travis.yml

@@ -1,7 +1,22 @@
+sudo: false
 language: go
 go:
- - 1.4
- - 1.5
- - tip
-install: go get -v ./github
-script: go test -v ./github
+  - 1.5.4
+  - 1.6.2
+  - tip
+matrix:
+  include:
+    - go: 1.4.3
+      script:
+        - go get -t -v ./...
+        - go test -v -race ./...
+  allow_failures:
+    - go: tip
+  fast_finish: true
+install:
+  - # Do nothing. This is needed to prevent default install action "go get -t -v ./..." from happening here (we want it to happen inside script step).
+script:
+  - go get -t -v ./...
+  - diff -u <(echo -n) <(gofmt -d -s .)
+  - go tool vet .
+  - go test -v -race ./...