Refactor to clense task name

When using Spring Boot, the id of the application context can be a colon
delimited string of values.  By default, Spring Cloud Task uses this
value as the task name.  However, when using JMX, this can cause issues.
This commit updates the SimpleTaskNameResolver to clense the colons and
replace them with underscores.

Resolves #280

spring-cloud-task-core/src/main/java/org/springframework/cloud/task/repository/support/SimpleTaskNameResolver.java

@@ -55,7 +55,7 @@ public class SimpleTaskNameResolver implements TaskNameResolver, ApplicationCont
 			return configuredName;
 		}
 		else {
-			return context.getId();
+			return context.getId().replace(":", "_");
 		}
 	}
 }