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 `springVersion` to check `BUILD-SNAPSHOT` release category on version
and enable `libs-snapshot` repository

To allow CI plan to build with SF snapshots it's just enough to configure the Gradle option
`-PspringVersion=4.0.1.BUILD-SNAPSHOT`