Override Jetty and Tomcat versions to maintain backwards compatibility

Spring Boot 1.2 has moved to used Jetty 9, Tomcat 8 and Servlet 3.1 by
default, while also continuing to support Jetty 8, Tomcat 7 and
Servlet 3.0. New major versions of Tomcat and Jetty is too big a change
for a minor release of Spring IO Platform.

This commit overrides the versions specified by Spring Boot so that
Spring IO Platform 1.1 provides the same Tomcat and Jetty versions as
1.0.x does. Spring IO Platform 2.0 will move to Tomcat 8 and Jetty 9.