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-schema-server/pom.xml

@@ -15,6 +15,10 @@
 			<groupId>org.springframework.cloud</groupId>
 			<artifactId>spring-cloud-stream</artifactId>
 		</dependency>
+		<dependency>
+			<groupId>org.springframework.boot</groupId>
+			<artifactId>spring-boot-starter-web</artifactId>
+		</dependency>
 		<dependency>
 			<groupId>org.springframework.boot</groupId>
 			<artifactId>spring-boot-starter-test</artifactId>