Glide requires $GOPATH, but Go 1.8 doesn't

[treeder]

Glide should use the default of $HOME/go if not set.

[mattfarina]

I wonder if we could use go env GOPATH if the GOPATH env var is empty. That may give us the right result.

[jmalloc]

I am using go env in Makefiles for similar reasons, and can confirm that it gives the correct results under 1.8 when $GOPATH is not set :)

[mattfarina]

There's a PR for this on #798

[comerc]

$ glide install
[ERROR]	$GOPATH is not set.

$ glide -v
glide version v0.12.3

go version go1.8.3 linux/amd64

Ubuntu 14.04.5 LTS