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.
diff --git a/pom.xml b/pom.xml
index 7522919..8e90ae8 100644
--- a/pom.xml
+++ b/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>