Add the issue tracker URL to error message
Add the issue tracker URL to error message

I think that it is user friendly that there is official issue tracker URL
in the error message when a bug occurs.

## Before

```console
Errors are usually caused by RuboCop bugs.
Please, report your problems to RuboCop's issue tracker.
Mention the following information in the issue report:
0.53.0 (using Parser 2.5.0.3, running on ruby 2.5.0 x86_64-darwin17)
```

## After

```console
Errors are usually caused by RuboCop bugs.
Please, report your problems to RuboCop's issue tracker.
https://github.com/bbatsov/rubocop/issues

Mention the following information in the issue report:
0.53.0 (using Parser 2.5.0.3, running on ruby 2.5.0 x86_64-darwin17)
```

vergenzt/rubocop,scottmatthewman/rubocop,bquorning/rubocop,sue445/rubocop,rrosenblum/rubocop,pocke/rubocop,deivid-rodriguez/rubocop,smakagon/rubocop,rrosenblum/rubocop,petehamilton/rubocop,petehamilton/rubocop,urbanautomaton/rubocop,tejasbubane/rubocop,bbatsov/rubocop,rrosenblum/rubocop,petehamilton/rubocop,vergenzt/rubocop,akihiro17/rubocop,mikegee/rubocop,jmks/rubocop,akihiro17/rubocop,tdeo/rubocop,palkan/rubocop,jfelchner/rubocop,jmks/rubocop,deivid-rodriguez/rubocop,palkan/rubocop,meganemura/rubocop,panthomakos/rubocop,akihiro17/rubocop,palkan/rubocop,tejasbubane/rubocop,sue445/rubocop,tdeo/rubocop,jmks/rubocop,meganemura/rubocop,panthomakos/rubocop,tejasbubane/rubocop,urbanautomaton/rubocop,jfelchner/rubocop,maxjacobson/rubocop,bquorning/rubocop,deivid-rodriguez/rubocop,panthomakos/rubocop,mikegee/rubocop,bquorning/rubocop,vergenzt/rubocop,bbatsov/rubocop,jfelchner/rubocop,scottmatthewman/rubocop,haziqhafizuddin/rubocop,pocke/rubocop,maxjacobson/rubocop,smakagon/rubocop,maxjacobson/rubocop,meganemura/rubocop,haziqhafizuddin/rubocop,sue445/rubocop,tdeo/rubocop,mikegee/rubocop