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

[hoatle]

now brew supports switch between different installed package versions, so we can switch vagrant versions easily.

[hoatle]

brew switch does not work with cask (vagrant, virtualbox), so it's still not convenient to work with different versions.

Currently, I will need to edit the cask tap to install the version I want to use, this is not easy and intuitive for most brew users.