Allow veeewee to install aside vagrant

Fixes JSON incompatibility errors with latest bundlers

Ex:

```bash
Resolving dependencies...
Bundler could not find compatible versions for gem "json":
  In Gemfile:
    vagrant (>= 0) ruby depends on
      json (< 1.8.0, >= 1.5.1) ruby

    veewee (>= 0) ruby depends on
      json (1.8.0)

```