Update Guava dependency

[Urokhtor]

Pact uses version 18.0 of Guava which was released in 2014. There are known vulnerabilities in this version (CVE-2018-10237 and CVE-2020-8908). While Pact is a library used in test runtime, it would still be good to keep the dependencies up to date.

This old version of Guava is causing headaches for people using dependency scanners because they get a lot of false positive vulnerabilities because of that.

[mefellows]

Thanks. Makes sense if it can be done, care to take a go at a PR to fix it?

[uglyog]

4.2.2 released