Travis: Add Go 1.8; use latest minor version.

Rename "tip" to "master", since it was renamed in Travis (but old name
is still supported for now).

Author: dmitshur

.travis.yml

@@ -1,12 +1,13 @@
 sudo: false
 language: go
 go:
-  - 1.6
-  - 1.7
-  - tip
+  - 1.6.x
+  - 1.7.x
+  - 1.8.x
+  - master
 matrix:
   allow_failures:
-    - go: tip
+    - go: master
   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).