Fix compilation errors due to enumerations

src/GomokuAI.java

@@ -109,10 +109,10 @@ public class GomokuAI extends Player {
 
             switch (this.color) {
 
-                case Color.WHITE:
+                case WHITE:
                     gomokuCell.setState(Color.BLACK);
                     break;
-                case Color.BLACK:
+                case BLACK:
                     gomokuCell.setState(Color.WHITE);
                     break;
                 default:

src/GomokuCell.java

@@ -239,9 +239,9 @@ public class GomokuCell {
     @Override
     public String toString() {
         switch (this.getState()) {
-            case Color.NIL:   return ".";
-            case Color.BLACK: return "X";
-            case Color.WHITE: return "O";
+            case NIL:   return ".";
+            case BLACK: return "X";
+            case WHITE: return "O";
             default:
                 throw new IllegalStateException("Unexpected value: " + this.getState());
         }