Allow building without Git

If Maven cannot find the git executable
the build fails.

Signed-off-by: Michael Sabin <m35@users.noreply.github.com>

airsonic-main/pom.xml

@@ -549,6 +549,7 @@
                 <configuration>
                     <doCheck>false</doCheck>
                     <doUpdate>false</doUpdate>
+                    <revisionOnScmFailure>Unversioned</revisionOnScmFailure>
                 </configuration>
             </plugin>
             <plugin>