Bug fix for reading references from GCS on Spark (#6070)

Bug fix for reading references from GCS on Spark

Without this fix, we could get a `java.nio.file.FileSystemNotFoundException: Provider "gs" not installed` when running on a cluster.

src/main/java/org/broadinstitute/hellbender/engine/spark/datasources/ReferenceFileSparkSource.java

@@ -50,7 +50,7 @@ public ReferenceFileSparkSource( final Path referencePath) {
 
     private synchronized Path getReferencePath() {
         if (null == referencePath) {
-            this.referencePath = Paths.get(referenceUri);
+            this.referencePath = IOUtils.getPath(referenceUri.toString());
         }
         return referencePath;
     }