Install git and mercurial

To make "go get" work in most cases you need either git or mercurial installed to clone the dependencies.

Now the golang class will install these but the requirement to update aptitude (on debian based machines) is not a concern for the golang class and so is handled outside of the class.

@sqreept Can you give this a try and see if this does what you want?

This solves both https://github.com/dcoxall/vagrant-golang/pull/6 and https://github.com/dcoxall/vagrant-golang/pull/5 whilst making an attempt at not being tied directly to debian (although there is still work to do to achieve this 100%)