Fix exception when threads write files to same dir

When Multiple FileAppenders attempt to write files to the same
nonexistent directory, concurrent threads could attempt to create
the directory at the same time, where only one succeeds, while
the others hit an IllegalStateException. This patch modifies
FileUtil.createMissingParentDirectories() to gracefully handle
existing directories (no exceptions thrown).

This patch also removes FileUtil.isParentDirectoryCreationRequired()
as this allows for race conditions between the existence-check and
the directory creation. Note that File.mkdirs() already performs
an existence-check.

Fixes LOGBACK-128
Merge pull request #203 from hazendaz/master

Update gmaven to gmavenplus plugin