build: use SNAPSHOT as default tag

Maven publishing treats the "SNAPSHOT" tag in a special way, marking that build as a snapshot type build.

gradle.properties

@@ -3,7 +3,7 @@ jmeVersion = 3.1.0
 # Version used for application and settings folder, no spaces!
 jmeMainVersion = 3.1
 # Version addition pre-alpha-svn, Stable, Beta
-jmeVersionTag = snapshot-github
+jmeVersionTag = SNAPSHOT
 
 # specify if JavaDoc should be built
 buildJavaDoc = true