Drop support for Go 1.8; Add support for Go 1.10 and make it the default.

It comes a bit late because previously we were hitting https://golang.org/issues/24285, and so have beeing waiting for slapos.core!42 (merged) to be deployed there on slapos nodes everywhere.

As of 9 Aug 2018 it seems that deployment has been completed.

@rafael, @alain.takoudjou, please confirm.