travis: Start building golang 1.8 as well

Require success from 1.7 since we'll probably move to it as the default
within six months.

.travis.yml

@@ -2,6 +2,7 @@ language: go
 go:
   - 1.6
   - 1.7
+  - 1.8
   - tip
 sudo: true
 dist: trusty
@@ -14,7 +15,7 @@ matrix:
   fast_finish: true
   allow_failures:
     - go: tip
-    - go: 1.7
+    - go: 1.8
 notifications:
   irc:
     channels: