gnu: go: update to 1.9.1 [security fixes].

See the release announcement and bug reports for more details about the
security issues fixed in this release:

https://groups.google.com/forum/m/#!topic/golang-nuts/sHfMg4gZNps
https://golang.org/issue/22125
https://golang.org/issue/22134

* gnu/packages/golang.scm (go-1.9): Update to 1.9.1.