fixed issue #4491 , upgrade druid 1.2.15

Author: agapple

pom.xml

@@ -262,7 +262,7 @@
             <dependency>
                 <groupId>com.alibaba</groupId>
                 <artifactId>druid</artifactId>
-                <version>1.2.13</version>
+                <version>1.2.15</version>
             </dependency>
             <dependency>
                 <groupId>com.lmax</groupId>