golang build failures

["James E. King, III" <jk...@apache.org>]

The root cause is someone added "import context" code to golang/mock which
broke it against older go releases (older than 1.7).

I have a fix for it bundled into THRIFT-4351 where I call "go get" twice
with a sed in between to force it to import golang.org/x/net/context
instead.

https://github.com/golang/mock/pull/118

The ubuntu-xenial official build docker image we use has go 1.6.2 in it
because that's what Ubuntu comes with.

This was the only build failure; once the next CI build passes I will be
merging this one in, which clears up a bunch of issues and adds a bit more
documentation for language support and what features each language provides
(but not a report of what features are cross-tested.. that is something we
want to generate programmatically).

- Jim