Upgrade hystrix to 1.4.15

fixes gh-534

pom.xml

@@ -27,7 +27,7 @@
 		<archaius.version>0.6.5</archaius.version>
 		<eureka.version>1.2.0</eureka.version>
 		<feign.version>8.10.0</feign.version>
-		<hystrix.version>1.4.13</hystrix.version>
+		<hystrix.version>1.4.15</hystrix.version>
 		<ribbon.version>2.1.0</ribbon.version>
 		<zuul.version>1.0.28</zuul.version>
 		<rxjava.version>1.0.11</rxjava.version>