Make CBLVersion internal

CBLVersion is not public API so making it internal.

Author: pasin

shared/src/main/java/com/couchbase/lite/CBLVersion.java

@@ -23,7 +23,7 @@
 
 import java.util.Locale;
 
-public class CBLVersion {
+class CBLVersion {
 
     static String userAgent = null;