Update web.xml

It's necessary to swap "load-on-startup" with "async-supported" because if not Eclipse will show this error (XML Problem):
cvc-complex-type.2.4.a: Invalid content was found starting with element 'load-on-startup'. One of '{"http://xmlns.jcp.org/xml/ns/javaee":run-as, "http://xmlns.jcp.org/xml/ns/javaee":security-role-ref, "http://xmlns.jcp.org/xml/ns/javaee":multipart-config}' is expected.	web.xml	/atmosphere-activemq-chat/src/main/webapp/WEB-INF	line 32	XML Problem
Merge pull request #35 from seirg/patch-1

Update web.xml