Upgrade spring dependency to mitigate security issues in spring

The 2.3 branch includes a dependency on Spring 3.0.3:

https://github.com/resteasy/Resteasy/blob/Branch_2_3/resteasy-spring/pom.xml#L64

This should be updated from 3.0.3 to >= 3.0.6 to avoid known security flaws:

http://support.springsource.com/security/cve-2011-2894