Update release script to merge to master after updating development version

When releasing new stable version of Confluence, pom versions in the stable branch are updated to the next development version (e.g. 5.5.5-SNAPSHOT). This will cause conflicts during the next merge to master. We need to modify the release script to perform a merge to master that will resolve these conflicts.

https://extranet.atlassian.com/display/~jwalton/2011/10/12/Maven+releases+treading+on+Git+merge%27s+toes contains a solution for this.

The change might be as simple as runnning "git merge -s ours release/5.5" in master branch after release task in https://confluence-bamboo.internal.atlassian.com/build/admin/edit/editBuildTasks.action?buildKey=CONFREL-CONFRELSTABLE2-RELEASEPREPARE. It might be a good idea to also add a pre-release step to merge release/5.5 to master so that no changes will be lost when using "ours" merge strategy.