Avoid integral division for Sorensen-Dice coefficient

Author: vpop

src/main/java/info/debatty/java/stringsimilarity/StringSet.java

@@ -54,6 +54,6 @@ public double sorensenDiceSimilarity(StringSet other) throws Exception {
             throw new Exception("Profiles were not created using the same kshingling object!");
         }
 
-        return 2 * this.vector.intersection(other.vector) / (this.vector.size() + other.vector.size());
+        return 2.0 * this.vector.intersection(other.vector) / (this.vector.size() + other.vector.size());
     }
 }