Vendor the mongo driver.
Add REVISION file to vendored mongo driver

So we can easily know what version is vendored
build_gopath.sh no longer needs to run go get.

All dependencies are now vendored.
Fix running tests against non-compiled router.
Merge remote-tracking branch 'origin/master' into vendor_mongo_driver
Merge pull request #50 from alphagov/vendor_mongo_driver

[Fixes #59399736] Vendor mongo driver