Use gom to manage dependencies

This uses https://github.com/mattn/gom to manage the external
dependencies.

Switching to this required figuring out the commit
numbers for the existing vendored things.  A diff between the resulting
_vendor directory and the existing vendor dir reveals that the only
differences are the SCM bits (.git directory etc).
Remove vendor directory

Dependencies are now handled by gom, so this is no longer needed.
Merge pull request #11 from alphagov/gom

 Use gom to manage dependencies