Update dependency on compile-testing to 0.18

[petedmarsh]

I ran into this library from this thread on compile-testing google/compile-testing#155 after having exactly the same problem (i.e. I'm using JUnit 5).

I have to admit that I didn't quite understand the difference between compile / implementation / api when I thought about updating the dependency on compile-testing (now I get it!) so this is relatively inconsequential. Still, it's probably a good idea to track compile-testing.

Thanks for the library!

[Kiskae]

After looking at the changes since 0.15 its probably a good idea to upgrade simply to avoid incompatibilities with future versions of google truth, I'll publish a new release soon.