[jetbrains] badge to show downloads count from jetbrains plugin repository v2

[platan]

This pull request is based on @mskonovalov's pull request #896. I've fixed issues reported by @paulmelnikow (badge format + service tests).

[platan]

I've forgotten about adding [jetbrains] to PR name (https://github.com/badges/shields/tree/master/service-tests#pull-requests). I'm not sure if Travis will see changed name. Could you please rerun Travis build?
By the way, maybe all service tests should be run when there is a PR for given branch? This would be less error prone.

[platan]

What do you think about this change?

[paulmelnikow]

This looks great! Thanks you for picking up the work from #896. I just restarted the Travis build and things look good.

If you merge master into this branch and fix the merge conflict, I will squash + merge this into master.

[platan]

I've merged master. Unfortunately CI build failed, because some GitHub's service tests failed. The same problem occurred in CRON build in master yesterday, 2 days ago, 3 days ago, and so on :-(

[platan]

I can see you're working intensively on GitHub rate limit problem #979

[paulmelnikow]

Much appreciated!