Add explicit dependencies to make Gradle pull in the desired version

Due to the lack of dependency management in Gradle a number of the
starters were pulling in old versions of spring-tx (and in some cases
spring-context-support as well) as the only dependency was a
transitive one that pulled in an older version.

This commit adds explicit dependencies on spring-context-support and
spring-tx where appropriate. These dependencies will specify Boot's
preferred version of Spring causing Gradle to do the right thing as it
prefers the latest version of a dependency when there is more than one
to choose from.

Fixes #1028