mongodb: head fixes

Formally asking for help here. Long story short, Mongodb is currently in the midst of a major code-base change involving moving a lot of the tools into an external directory and using Google's Golang to retrieve and build them.

See [here](https://jira.mongodb.org/browse/SERVER-15555) and [here](https://jira.mongodb.org/browse/SERVER-15806) to get up to speed on upstream.

You can see instructions for which order to install the tools [here](https://github.com/mongodb/mongo-tools).

My current error with this build is:

```
Error: undefined method `go_resource' for #<HeadSoftwareSpec:0x007ff16b29f8b0>
```

Thoughts? Can we not yet specify a Go build to the HEAD only? Or am I doing something else wrong?
No worries. We've removed the ``` HEAD ``` for now to stop people running headlong into failure, but we'll add it back in once we've reached some easier solution here.