Enable parallel build to reduce build times

[TWiStErRob]

gradlew build (including check/test) takes 1 minute 17 seconds with current master. Adding this single line decreases that time to just 14 seconds.

Measured with

gradlew clean build check -x gradle-maven-test:check -x gradle-maven-test:test -x gradle-maven-test:build

Timings don't include any mvnw calls. I don't have that on my PATH.

0:14 build scan: https://gradle.com/s/sw2cncnoyeips 1:17 build scan: https://gradle.com/s/ym2ae3e2qviaa