Make heroku-keepalive optional

[technicalpickles]

Suggested in https://github.com/github/hubot/issues/827 by @mal

We have to run our Hubot on our own hardware due to security policy, unfortunately this means we get messages about missing Heroku URLs when starting up.

I've removed the plugin from the package.json and external-scripts.json but it would good to have it as an option at creation time.

[iancward]

@technicalpickles It looks like heroku-keepalive is still in the generator script: https://github.com/github/generator-hubot/blob/97746f1fe73df1aa00859c9a8c9ba5fd4fedae9e/generators/app/index.js#L139

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[iancward]

Hi, This is still an issue.

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[iancward]

Hi, This is still an issue.

[joeyguerra]

no longer a problem.