INT-3274: build.gradle: BUILD-SNAPSHOT condition

JIRA: https://jira.springsource.org/browse/INT-3274

* Change the `libs-snapshot` repository to `libs-milestone`
* Add condition on `project.version` to check `BUILD-SNAPSHOT` release category
and enable `libs-snapshot` repository
* Remove redundant directories from `mqtt` module
* and one more project property for Spring AMQP version

To test it om master:
1. `gradle.properties -> version=4.0.0.M3`
2. `gradlew clean test --refresh-dependencies -PspringAmqpVersion=1.2.1.RELEASE`