Add 'pom.xml.versionsBackup'

'mvn versions:use-latest-versions' generates 'pom.xml.versionsBackup' file, because '-DgenerateBackupPoms=true' is default; see http://mojo.codehaus.org/versions-maven-plugin/use-latest-versions-mojo.html#generateBackupPoms

Maven.gitignore

 target/
 pom.xml.tag
 pom.xml.releaseBackup
+pom.xml.versionsBackup
 pom.xml.next
 release.properties