Enforce matching version in Java and C#.

Change-Id: I7f1f12f2f97e5227e0dabc2965ce66a6d41c229c

java/com/google/flatbuffers/Constants.java

@@ -39,6 +39,14 @@ public class Constants {
     static final int FILE_IDENTIFIER_LENGTH = 4;
     /** The number of bytes in a size prefix. */
     public static final int SIZE_PREFIX_LENGTH = 4;
+    /** A version identifier to force a compile error if someone
+    accidentally tries to build generated code with a runtime of
+    two mismatched version. Versions need to always match, as
+    the runtime and generated code are modified in sync.
+    Changes to the Java implementation need to be sure to change
+    the version here and in the code generator on every possible
+    incompatible change */
+    public static void FLATBUFFERS_1_11_1() {}
 }
 
 /// @endcond