npm contentUri should always end in slash

In order for npm cli version 2.1.7 or higher to be happy, registry url passed to it must end in slash. For consistency, I propose to change the contentUri method to enforce this. This helps the PublishIT we have to pass the correct registry url to the npm cli.

See the mentioned npm issue I filed to see if this was a regression in the npm cli.
Revert "npm contentUri should always end in slash"

This reverts commit 5fbb12de80769a8bc781412d9fae1963e5007355.
change PublishIT to work around npm registry url bug

both non slash and ending slash *should* work and I was too lazy to check the version number of nom to preserve old behaviour
Merge pull request #928 from sonatype/nexus-2.11.x-npm-contenturi-slash

npm contentUri should always end in slash for publish
prepare the ITs test suite to work with Rubygems version 2.4.5

using the gem command line tools is tricky since they keep state between the
execution of commands. this patch reduces the impact of this caching on the ITs.