Switch from `third_party.go` to `gom` for dependencies, fixes #80

This change pins the dependency versions we're using. It also removes
the alphagov fork of the redis library we were using because we can
reference the specific version we want.
Merge pull request #81 from alphagov/use-gom-for-pinned-dependencies

Switch from `third_party.go` to `gom` for dependencies, fixes #80