Spring 4 support

Added explicit support for Spring 4.x as TestContext is now an interface and not a class as in earlier versions. See https://jira.springsource.org/browse/SPR-7692 for more information. Unfortunately, I could not find a better way than making a copy of flyway-spring-test.

Please note that I had reversed the version number to 2.3 to align with Flyway.