[build] Polish repo.spring.io repositories, remove libs prefix

This commit removes the `libs-` prefix to repo.spring.io repositories,
and removes the reference to the `release` one (all releases should be
taken from maven central from now on)

This is a cherry-pick of the same change that was done on 1.2.x as
commit 943f353.

Author: simonbasle

build.gradle

@@ -16,6 +16,7 @@
 
 buildscript {
 	repositories {
+		jcenter()
 		maven { url "https://repo.spring.io/plugins-release" }
 	}
 	dependencies {
@@ -67,10 +68,9 @@ configure(allprojects) { project ->
 	group = 'io.projectreactor.rabbitmq'
 
 	repositories {
-		maven { url 'https://repo.spring.io/libs-release' }
-		maven { url 'https://repo.spring.io/libs-snapshot' }
 		mavenCentral()
 		jcenter()
+		maven { url 'https://repo.spring.io/snapshot' }
 	}
 
 	apply plugin: 'maven'