Allow the flyway-gradle-plugin to compile with 1.6 bytecode

This should resolve #779. Curiously enough, this affects OSX builds of IntelliJ when using the Gradle Tooling API inside of IntelliJ since (as of this writing) it still uses the Mac Java 6 runtime under the covers to execute Gradle. Whenever `flyway` is included as a plugin dependency from a Gradle build, it will choke on the 1.7 compiled bytecode in the plugin when trying to parse the build.