Lock guava to 20.0

[carlossg]

Otherwise 21.0 causes
java.lang.NoSuchMethodError: com.google.common.util.concurrent.MoreExecutors.sameThreadExecutor()Lcom/google/common/util/concurrent/ListeningExecutorService;

Broken with #62

[oleg-nenashev]

Could we just use guava from the core?

[jglick]

I suspect what you meant to do was to use an <exclusion> of guava from io.findify:s3mock_2.12…?

[jglick]

cf. #44

[carlossg]

I don't know if s3mock needs any specific version of guava, at least it works with 20. May not work with previous versions

[jglick]

It is just that we care a lot about jclouds and the AWS SDK being bundled with a preferred version of Guava; and not at all about S3Mock getting a particular version—either the tests pass or they do not. Using exclusion assures us that the version is picked up from the right place even if we update the libs being used at runtime.