Add go1.8 to CI tests (#897)

* Add go1.8 to CI tests

What
===
Add go1.8 to CI tests.

Why
===
It's the latest Go release and we should be ensuring tests pass on it.

* Remove go1.6 from CI tests.

What
===
Remove go1.6 from CI tests.

Why
===
The gocql README calls out that the last two major versions will be
supported. The last two major versions are go1.7 and go1.8.

.travis.yml

@@ -25,8 +25,8 @@ env:
       AUTH=false
 
 go:
-  - 1.6
   - 1.7
+  - 1.8
 
 install:
   - pip install --user cql PyYAML six