* Explicitly check go version in build

Several GH issues have been opened by people trying to use an older
version of Go to build Vault (e.g., #3307 is the most recent). This adds
an explicit check to the build to hopefully make it more clear to users
in the future.

* Also add checking for go patch version

* Up minimum go version

And fix a comment

* Bump travis to go1.9.1