Add Git dependency to plugin
Fix plugin install

The `test` command in the `onlyif` and `unless` directives needs a
specified path.
Add Git dependency to build
Add build-essential dependency to build
Enforce explicit path to rbenv in build execs
Set $RBENV_ROOT in relevant build execs

Exec commands are run in subshells, so exported variables are not
persistent. Therefore, environment variables must be explicitly set when
needed.
Update dependencies for build
Ensure build-essential is installed in build test
Remove external dependencies from build

The `rbenv` module probably shouldn't be responsible for declaring
external dependencies, despite [this commit][1]. The manner in which
these dependencies are managed should be at the user's discretion.

[1]: http://git.io/Zm39Og
Enable CFLAGS to be configurable for build
Updated rbenv::build, rbenv:gem. Release 0.2.0 Fixes #2 #4
* removed all external dependencies (git, etc)
* add new cflags parameter (thanks to @thaumazein)
* updated all tests so they pass with Vagrantfile