Use parallel build options (#2808)
Use parallel build options (#2808)

Add parallel option for gradle builds.

Probably it can improve #2680 after more and more modules introduction.

Timing examples:
* [Without parallel - ubuntu 8 - 4m 23s](https://github.com/detekt/detekt/pull/2779/checks?check_run_id=778117131).
* [With parallel - ubuntu 8 - 1m 21s](https://github.com/detekt/detekt/pull/2808/checks?check_run_id=778157588).
* [Without parallel - macos 8 - 6m 14s](https://github.com/detekt/detekt/pull/2779/checks?check_run_id=778117167).
* [With parallel - macos 8 - 3m 38s](https://github.com/detekt/detekt/pull/2808/checks?check_run_id=778157753).
arturbosch/detekt,arturbosch/detekt,arturbosch/detekt