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

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

* 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=3.2.7.BUILD-SNAPSHOT`