Change JUnit @Test expected parameter with ExpectedException @Rule in triangle exercise [Fix #268]

exercises/triangle/src/test/java/TriangleTest.java

 import org.junit.Test;
 import org.junit.Ignore;
-import org.junit.rules.TestWatcher;
+import org.junit.Rule;
+import org.junit.rules.ExpectedException;
 
 import static org.junit.Assert.assertEquals;