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.