Add explict spring-context dependency to spring-rabbit

spring-rabbit includes a number of classes with compile dependencies
upon spring-context, for example RabbitAdmin. It's currently only
getting spring-context transitively via its optional dependency on
spring-retry.

This causes a problem when spring-rabbit is depended upon as it can
lead to spring-retry's version of spring-context being used. This is
3.0.5.RELEASE which does not match the versions of the other spring
modules that spring-rabbit directly depends upon.

This commit adds a direct compile dependency upon spring-context
which better expresses the project's dependencies and ensures that a
matching and up-to-date version of spring-context is used.