Travis: Update.

Use container-based builds.

Build for tip (with allowed failure and fast finish).

.travis.yml

@@ -1,11 +1,18 @@
+sudo: false
+addons:
+  apt_packages:
+    - libgl1-mesa-dev
+    - xorg-dev
 language: go
 go:
-  - 1.5
-before_install:
-  - sudo apt-get update
-  - sudo apt-get install -qq libgl1-mesa-dev xorg-dev
+  - 1.5.2
+  - tip
+matrix:
+  allow_failures:
+    - go: tip
+  fast_finish: true
 install:
-  - go get golang.org/x/tools/cmd/vet
+  - # 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 ./v3.1/...
   - diff -u <(echo -n) <(gofmt -d -s .)