GH-1175 Removed dependency on spring-boot-starter-web

- Removed spring-boot-starter-web dependency from the core
- Polished AggregateApplicationBuilder to ensure it thriows meaningful exception when web is enabled but 'spring-boot-starter-web' is not on the classpath
- fixed Aggregator tests to defualt to no-web
- added spring-boot-starter-web to schema projects

Resolves #1175

spring-cloud-stream-test-support/src/test/java/org/springframework/cloud/stream/test/aggregate/main/AggregateWithMainTest.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2016-2017 the original author or authors.
+ * Copyright 2016-2018 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -52,7 +52,7 @@ public class AggregateWithMainTest {
 	@Test
 	public void testAggregateApplication() throws InterruptedException {
 		// emulate a main method
-		ConfigurableApplicationContext context = new AggregateApplicationBuilder(MainConfiguration.class)
+		ConfigurableApplicationContext context = new AggregateApplicationBuilder(MainConfiguration.class).web(false)
 				.from(UppercaseProcessor.class).namespace("upper")
 				.to(SuffixProcessor.class).namespace("suffix")
 				.run("--spring.cloud.stream.bindings.input.contentType=text/plain","--spring.cloud.stream.bindings.output.contentType=text/plain");