Publish to Maven Central

[coveralls]

Coverage remained the same at 96.183% when pulling d7b5cc0e1290d3bf8f81b5a43d61ced5069bc8bd on publish-m2_v3 into bf19039387034ee9c83bba896e963da6471efd3e on master.

[jmewes]

Current benefits:

In the previous WIP pull request, the publishToSonatype Gradle task was successfully executed. Also the signing worked.

See https://github.com/ePages-de/restdocs-wiremock/pull/72#issuecomment-796837414

Current limitations:

• The documentation in the README is not yet updated. I want to do this once the library is actually available on Maven Central.
• The release needs to be closed and released in the Sonatype UI. It could be automated but it is recommended by the "publish-plugin" README to start with the manual process.
• The publishToSonatype was executed successfully but created no staging repo yet, because this will only happen if the task is run on a commit with a tag.

Next steps:

Once this PR is merged I want to create a release with the version number "0.8.0". This should trigger a Travis build and create staging repository at Sonatype. This I want to manually close and release which should make restdocs-wiremock available on Maven Central.