Fix issues with the release process

[mccartney]

When trying to release 0.2.12 sbt ++publishedSigned raised the following errors:

[...]
[error]     source     : /home/mccartney/git/slack-scala-client/target/scala-2.11/slack-scala-client_2.11-0.2.12-javadoc.jar.asc
[error]     destination: https://oss.sonatype.org/service/local/staging/deploy/maven2/com/github/slack-scala-client/slack-scala-client_2.11/0.2.12/slack-scala-client_2.11-0.2.12-javadoc.jar.asc
[error] If you have a staging repository that has failed, drop it and start over.
[error] Otherwise fix the double publishing, or relax the setting as follows:
[error]     publishConfiguration := publishConfiguration.value.withOverwrite(true)
[error]     publishLocalConfiguration := publishLocalConfiguration.value.withOverwrite(true)
[error] 
[error] If you have a remote cache respoitory, you can enable overwriting as follows:
[error]     pushRemoteCacheConfiguration := pushRemoteCacheConfiguration.value.withOverwrite(true)
[error] Total time: 36 s, completed 02-Jan-2021 17:58:37

but apparently all the rest went fine.

I suspect it tried to upload the javadoc.jar.asc the second time for some reason.
My gut feeling is that maybe it was "main code" and "test code" javadoc instead of one?