* 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
Joel Thompson