travis: build Go 1.7 as well, removed by revert

Signed-off-by: Stephen J Day <stephen.day@docker.com>
diff --git a/.travis.yml b/.travis.yml
index efed8ea..804c569 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,6 +1,7 @@
 language: go
 go:
   - 1.6
+  - 1.7
   - tip
 install:
   - go get -t ./...