Document potential issue with installing vagrant-hostupdater

[iros]

My initial attempts to set it up failed because of my local nokogiri version.
I had to run this first: sudo gem install nokogiri -v '1.6.6.2'

Not sure if it's worth documenting, but maybe?

[iros]

This issue is documented: hashicorp/vagrant#3769
I was only able to solve it by upgrading vagrant to 1.7.2.

[tkellen]

@cowboy Seems we should make the latest vagrant a prereq?

[cowboy]

Yeah, the Dependencies documentation already says "All versions should be the latest available, unless otherwise noted."

Is there anything else we should do here?

[tkellen]

Making this bold might be useful.

All versions should be the latest available, unless otherwise noted.

It would create an annoying maintenance burden, but it might not hurt to actually put the known working versions in so people have something to compare to.

[cowboy]

I've made it very explicit that the latest versions of all dependencies should be installed in the README.