Replace Clover with JaCoCo for code coverage

[pjeli]

Clover requires a license to use and is complicated to set-up.

JaCoCo is free and very simple to set-up now that we've gradle-ized.

We should make the switch!

[pjeli]

All we need to do to support JaCoCo is add it as a gradle plugin to giraffa-core; literally a one-line change.

Then we just run the following commands: ./gradlew clean test jacocoTestReport

And in giraffa-core/build/reports/ there will be a new jacoco directory which will have an HTML generated report.

[milandesai]

+1

[pjeli]

Thanks @milandesai.

Committed as: 2a38df17013132352d042270f3cc67a160281dda.