Use Go 1.9.4

[prymitive]

This switches Travis and Docker builds to use latest stable Go release

[prymitive]

I think that solaris builds are failing because of the new cgo restrictions placed in Go 1.9.4 - golang/go#23672, but it seems that there's a change targeting Go 1.10 that should allow it again - https://go-review.googlesource.com/c/go/+/94675

[terinjokes]

@prymitive Worth disabling Solaris builds in bakelite until you target 1.10 then?

[prymitive]

1.10 is already out, need to check if it was included

[terinjokes]

@prymitive So it did come out a few hours ago.

[prymitive]

1.10 seems to crosscompile just fine, I'll re-open with 1.10 once gimme and Go docker images are ready for 1.10

[prymitive]

Followup in #236