create and maintain brew tap to install specific vagrant versions

[hoatle]

So that Mac users could use the specified vagrant version to avoid incompatibility issues:

• Feature: Support the installation of specific versions Homebrew/homebrew-cask#471
• https://github.com/Homebrew/brew/blob/master/docs/How-to- -and-Maintain-a-Tap.md

$ brew tap teracyhq/homebrew-tap

and install the specified software versions (virtualbox, vagrant) or the latest ones.

This is required as vagrant 1.9 has problems, we need to use vagrant 1.8 instead for now. When 1.9 is stable enough, we'll upgrade it later with enough testing.

[hoatle]

@duc4nh could help on this