Bump version down to 3.1.99-SNAPSHOT, next release is 3.2.0

The changes in SLING-8997 are significant. However, I don't expect anyone
to actually import the packages, therefore not bumping the major component
of the version.

pom.xml

@@ -29,7 +29,7 @@
     </parent>    
 
     <artifactId>org.apache.sling.dynamic-include</artifactId>
-    <version>4.0.0-SNAPSHOT</version>
+    <version>3.1.99-SNAPSHOT</version>
 
     <name>Apache Sling Dynamic Include</name>
     <description>Dynamic Include filter for Apache Sling</description>