Upgrade to Gradle 4.9; fix RotatingServersTests

Stop the flow before examining the stream to avoid `ConcurrentModificationException`.

**cherry-pick to 5.0.x**

# Conflicts:
#	gradle/wrapper/gradle-wrapper.jar
#	gradle/wrapper/gradle-wrapper.properties

Author: garyrussell

gradle/wrapper/gradle-wrapper.jar

@@ -1,5 +1,5 @@
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-4.6-bin.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-4.9-bin.zip
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists

gradle/wrapper/gradle-wrapper.properties

@@ -41,6 +41,7 @@
 import org.springframework.integration.dsl.IntegrationFlows;
 import org.springframework.integration.dsl.channel.MessageChannels;
 import org.springframework.integration.dsl.Pollers;
+import org.springframework.integration.dsl.StandardIntegrationFlow;
 import org.springframework.integration.file.remote.aop.RotatingServerAdvice;
 import org.springframework.integration.file.remote.aop.RotatingServerAdvice.KeyDirectory;
 import org.springframework.integration.file.remote.session.CachingSessionFactory;
@@ -97,6 +98,7 @@ public void testStandard() throws Exception {
 		AnnotationConfigApplicationContext ctx = new AnnotationConfigApplicationContext(StandardConfig.class);
 		StandardConfig config = ctx.getBean(StandardConfig.class);
 		assertThat(config.latch.await(10, TimeUnit.SECONDS)).isTrue();
+		ctx.getBean(StandardIntegrationFlow.class).stop();
 		List<Integer> sfCalls = config.sessionSources.stream().limit(17).collect(Collectors.toList());
 		assertThat(sfCalls).containsExactly(1, 1, 1, 2, 2, 2, 3, 3, 3, 1, 1, 2, 2, 3, 3, 1, 1);
 		File f1 = new File(tmpDir + File.separator + "standard" + File.separator + "f1");
@@ -114,6 +116,7 @@ public void testFair() throws Exception {
 		AnnotationConfigApplicationContext ctx = new AnnotationConfigApplicationContext(FairConfig.class);
 		StandardConfig config = ctx.getBean(StandardConfig.class);
 		assertThat(config.latch.await(10, TimeUnit.SECONDS)).isTrue();
+		ctx.getBean(StandardIntegrationFlow.class).stop();
 		List<Integer> sfCalls = config.sessionSources.stream().limit(17).collect(Collectors.toList());
 		assertThat(sfCalls).containsExactly(1, 1, 2, 2, 3, 3, 1, 1, 2, 2, 3, 3, 1, 1, 2, 2, 3);
 		File f1 = new File(tmpDir + File.separator + "fair" + File.separator + "f1");
@@ -132,6 +135,7 @@ public void testVariableLocalDir() throws Exception {
 		AnnotationConfigApplicationContext ctx = new AnnotationConfigApplicationContext(VariableLocalConfig.class);
 		StandardConfig config = ctx.getBean(StandardConfig.class);
 		assertThat(config.latch.await(10, TimeUnit.SECONDS)).isTrue();
+		ctx.getBean(StandardIntegrationFlow.class).stop();
 		List<Integer> sfCalls = config.sessionSources.stream().limit(17).collect(Collectors.toList());
 		assertThat(sfCalls).containsExactly(1, 1, 1, 2, 2, 2, 3, 3, 3, 1, 1, 2, 2, 3, 3, 1, 1);
 		File f1 = new File(tmpDir + File.separator + "variable" + File.separator + "foo" + File.separator + "f1");
@@ -148,6 +152,7 @@ public void testVariableLocalDir() throws Exception {
 	public void testStreaming() throws Exception {
 		AnnotationConfigApplicationContext ctx = new AnnotationConfigApplicationContext(StreamingConfig.class);
 		StandardConfig config = ctx.getBean(StandardConfig.class);
+		ctx.getBean(StandardIntegrationFlow.class).stop();
 		assertThat(config.latch.await(10, TimeUnit.SECONDS)).isTrue();
 		List<Integer> sfCalls = config.sessionSources.stream().limit(17).collect(Collectors.toList());
 		// there's an extra getSession() with this adapter in listFiles