Make raindrops use instance method

Fixes issue #352

exercises/raindrops/src/example/java/Raindrops.java → exercises/raindrops/src/example/java/RaindropConverter.java

-public class Raindrops {
+public class RaindropConverter {
 
-    public static String convert(int number) {
+    public String convert(int number) {
         String result = "";
 
         if (number % 3 == 0) {

exercises/raindrops/src/test/java/RaindropsTest.java → exercises/raindrops/src/test/java/RaindropConverterTest.java

 import static org.junit.Assert.assertEquals;
 
 @RunWith(Parameterized.class)
-public class RaindropsTest {
+public class RaindropConverterTest {
 
     private int inputNumber;
     private String outputFromRaindropConversion;
         });
     }
 
-    public RaindropsTest(int inputNumber, String outputFromRaindropConversion) {
+    public RaindropConverterTest(int inputNumber, String outputFromRaindropConversion) {
         this.inputNumber = inputNumber;
         this.outputFromRaindropConversion = outputFromRaindropConversion;
     }
 
     @Test
     public void test() {
-        assertEquals(outputFromRaindropConversion, Raindrops.convert(inputNumber));
+        assertEquals(outputFromRaindropConversion, new RaindropConverter().convert(inputNumber));
     }
 }