Card origin_runtime_21 - Enable BUNDLE_WITHOUT env variable for Ruby

This PR will allow Ruby cartridge users to set `BUNDLE_WITHOUT` environment variable via using `rhc env set` to specify a Gemfile groups they don't want to install during application deployment.

For instance, if to prevent installing 'xapian' in Redmine when users have 'dmsf' plugin installed as a GIT submodule, all you need to do is:

`rhc env set BUNDLE_WITHOUT="xapian"`

and then do a `git push` as usuall.

I tested using BUNDLE_WITHOUT just as an env variable but for some reason, this variables is not picked up by build() function, so I added it explicitely using `--without` option. 

`BUNDLE_WITHOUT` is a standard bundler variable defined in (bundle-config)[http://bundler.io/v1.3/man/bundle-config.1.html] man page:

```
The canonical form of this configuration is "without". To convert the canonical form
to the environment variable form, capitalize it, and prepend BUNDLE_.
The environment variable form of "without" is  BUNDLE_WITHOUT.
```