* Update go-bootstrap package
The last C based Go src tree was the 1.4 series. For a while they
were cutting new releases so that people could bootstrap from a C only
system. Now they're recommending that you either use the release-1.4
branch or that you use a date-stamped tarball that they'll produce on
an as-needed basis.
There are several issues that keep 1.4.2 from building on a CentOS 7
system.
I've switched to the date based tarball.
The cgo bits were also mis-behaving, but they're not needed for the
bootstrapping task so I've set an environment variable that disables
them.
Details [on the install-from-source
page](https://golang.org/doc/install/source#go14) and these issues:
- https://github.com/golang/go/issues/17545
- https://github.com/golang/go/issues/16352.
* Update go package
Switched from pulling from the git repository to using the source
tarballs and added digest values.
Added support for 1.7.3, continued supporting 1.6.2, including patches
for a couple of problems (details in
[17545](https://github.com/golang/go/issues/17545) and
[17986](https://github.com/golang/go/issues/17986).
Dropped support for 1.5.4 and 1.4.2 because they no longer pass their
tests and the patches above to not apply.
