difference-of-squares: add version to test suite

Goal: synchronize version numbers of generic test suites in x-common
and concrete test suites in the individual tracks.

exercises/difference-of-squares/src/test/java/DifferenceOfSquaresCalculatorTest.java

 
 import static org.junit.Assert.assertEquals;
 
+/*
+ * version: 1.1.0
+ */
 public final class DifferenceOfSquaresCalculatorTest {
 
     private DifferenceOfSquaresCalculator calculator;