Previously, the only starter that provided validation was
spring-boot-starter-web which included Hibernate Validator and
Tomcat's EL implementation. This left users writing non-web
applications to figure out the dependencies for themselves. They would
sometimes run into difficulties as Hibernate Validator's need for an
EL implementation would trip them up.

This commit adds a new starter, spring-boot-starter-validation,
which provides both Hibernate Validator and Tomcat's EL
implementation. spring-boot-starter-web has been updated to depend on
this starter rather than depending on Hibernate Validator directly.

Closes gh-2678