Make runner commands use bundle exec (if a Gemfile is present)

[judithroth]

When whenever registers a runner command, it doesn’t use bundle exec in the resulting crontab. This may cause errors like this:
'gem_original_require': no such file to load -- bundler/setup (LoadError)

This patch makes whenever runner commands use "bundle exec" when there's a Gemfile present (which indicates use of bundler).

[ajsharp]

Bump

[benlangfeld]

Merged as 9f37076. Thank you for the contribution @judithroth!

[javan]

I believe this will construct bundle exec bin/rails runner when both bundler and bin/rails are present, which makes using the bin/rails binstub redundant. I think it'll work fine, just making a note of it.

[benlangfeld]

It is indeed redundant @javan, but does work. I figured it was better to resolve the bug quickly, and then come up with a consistent answer to binstubs vs bundle exec at lower priority :)

[benlangfeld]

Also note some people having trouble executing their binstubs directly at #555. We'll have to take that into account if we want to smooth this out.