Upgrade to Hibernate Validator 5.0.2

Update Hibernate Validator dependency to 5.0.2 and also include
Tomcat Jasper dependency. Jasper is required as it provides the
default ExpressionFactory implementation class
(org.apache.el.ExpressionFactoryImpl) which is indirectly
instantiated by Hibernate Validator.

Fixes gh-173
Upgrade to Tomcat 7.0.50

Upgrade to Tomcat 7.0.50, working around the potential
NullPointerException by also adding dependencies to
tomcat-embedded-jasper (which is now also required for Hibernate
Validator 5.0, see commit 377953babd3dcd15b6a29b13ae7882e88253a926)

Fixes gh-245