(#902) Check for github_changelog_generator in proper bundler context

[scotje]

Previously, the code was checking for the github_changelog_generator gem
in the PDK's execution environment, but then invoking the gem in the
module's execution environment. This change updates the code to check
for the gem in the module's execution environment by invoking bundle show.

[coveralls]

Coverage increased (+0.003%) to 92.649% when pulling f54ad9e on scotje:fix_changelog_check into c178798 on puppetlabs:master.

[kenyon]

This PR just changed the error message slightly, but as a user, still isn't very helpful. The Do you want to run the automatic changelog generation? question is a standard part of pdk release prep`, so why not include the gem in PDK by default, and provide some documentation on how to use this?

Initially running pdk release prep I got pdk (ERROR): Unable to generate the changelog as the github_changelog_generator gem is not included in this module's Gemfile. So I did pdk bundle add github_changelog_generator. Next run of pdk release prep I got

/home/kenyon/.pdk/cache/ruby/2.7.0/gems/github_changelog_generator-1.15.2
rake aborted!
Set CHANGELOG_GITHUB_TOKEN environment variable eg 'export CHANGELOG_GITHUB_TOKEN=valid_token_here'
/home/kenyon/git/puppet-bind/Rakefile:48:in `block in <top (required)>'
/home/kenyon/.pdk/cache/ruby/2.7.0/gems/github_changelog_generator-1.15.2/lib/github_changelog_generator/task.rb:42:in `define'
/home/kenyon/.pdk/cache/ruby/2.7.0/gems/github_changelog_generator-1.15.2/lib/github_changelog_generator/task.rb:36:in `initialize'
/home/kenyon/git/puppet-bind/Rakefile:47:in `new'
/home/kenyon/git/puppet-bind/Rakefile:47:in `<top (required)>'
/opt/puppetlabs/pdk/share/cache/ruby/2.7.0/gems/rake-12.3.3/exe/rake:27:in `<top (required)>'
/opt/puppetlabs/pdk/private/ruby/2.7.2/bin/bundle:23:in `load'
/opt/puppetlabs/pdk/private/ruby/2.7.2/bin/bundle:23:in `<main>'
(See full trace by running task with --trace)
pdk (ERROR): Error generating changelog:

🤷

[jpogran]

@kenyon I referenced your comment in a new issue so that this isn't lost in a pull request closed a long time ago. Please use that issue for a further discussion