Remove duplicate jaxb-core dependency

[cbeach47]

In uPortal-start's .gradle/tomcat/webapps/WebProxyPortlet/WEB-INF/lib directory, there are duplicate jaxb-core jars with different versions. Maven isn't flagging them as duplicates since they are coming from different packages.

We should remove one of duplicate dependencies, possibly by upgrading jaxb in one of the dependencies (as opposed to excluding a dependency via WebproxyPortlet's maven build).

• com.sun.xml.bind > jaxb-core-2.2.7.jar
• org.glassfish.jaxb > jaxb-core-2.3.0.1.jar

These are coming from:
org.jasig.portal:uPortal-spring:jar:5.15.1 >
org.jasig.portal:uPortal-layout-core:jar:5.15.1 >
org.apereo.service.persondir:person-directory-impl:jar:1.8.11 >
com.sun.xml.bind:jaxb-impl:jar:2.2.7
org.jasig.cas.client:cas-client-core:jar:3.6.4