Start using dep for dependency management

[joelnb]

Given that the glide authors have added this to the README:

The Go community now has the dep project to manage dependencies. Please consider trying to migrate from Glide to dep. If there is an issue preventing you from migrating please file an issue with dep so the problem can be corrected. Glide will continue to be supported for some time but is considered to be in a state of support rather than active feature development.

it seems like it would be a good time to move to dep for dependency management.

I removed the glide steps from the Makefile when making this change (rather than replacing them with a dep equivalent) because given that the vendor directory is part of the repo it doesn't seem that important.