New flag to prevent loading `bundle exec`

New way of skipping Bundler.setup, as documented here:
https://github.com/mpapis/rubygems-bundler

If you use NOEXEC=0 as, you get this deprecation warning:
`Warning, 'NOEXEC' environment variable is deprecated, switch to 'NOEXEC_DISABLE=1'`
Merge pull request #138 from slothbear/patch-2

New flag to prevent loading `bundle exec`