Add a script to check the Go version before building dex. This
gives a nice error message rather than just failing to compile.

With changes:

    $ go version
    go version go1.6.4 linux/amd64
    $ make
    ERROR: dex requires Go version 1.7+. Please update your Go installation: https://golang.org/dl/
    Makefile:93: recipe for target 'check-go-version' failed
    make: *** [check-go-version] Error 2

Checks only added for building the actual binary, not tests, since
this is aimed at users just starting off with the project.