(PDK-1187) Don't override bundler path on gem installs

[rodjek]

If PDK has been installed as a gem (for example, specified in a module's Gemfile), don't try to force a particular bundler path as this will usually conflict with the user specified path and break their workflow.

PS C:\Users\tsharpe\puppetlabs-windows> bundle exec pdk build
pdk (INFO): Building puppetlabs-windows version 5.0.0
pdk (INFO): Build of puppetlabs-windows has completed successfully. Built package can be found here: C:/Users/tsharpe/puppetlabs-windows/pkg/puppetlabs-windows-5.0.0.tar.gz
PS C:\Users\tsharpe\puppetlabs-windows> bundle exec puppet --version
6.0.4
PS C:\Users\tsharpe\puppetlabs-windows> cat .\.bundle\config
---
BUNDLE_PATH: ".bundle/gems"

[coveralls]

Coverage decreased (-0.01%) to 92.985% when pulling 66137ad on rodjek:pdk-1187 into 5158bfd on puppetlabs:master.