travis: use .x syntax to ensure latest 1.7

1.8.x doesn't work yet (it can't pick-up pre-releases).

Author: Francisco Souza

.travis.yml

@@ -3,6 +3,6 @@ install: make testdeps
 script: make coverage
 after_success: bash <(curl -s https://codecov.io/bash)
 go:
-  - 1.7.4
+  - 1.7.x
   - 1.8beta2
   - tip