Making actuator and web optional dependencies

without this change we're explicitly requiring our users to take both web and actuator into their classpath. That makes little sense since not every app is a web app.
with this change we're making those deps optional

fixes #524

spring-cloud-starter-sleuth/pom.xml

@@ -20,14 +20,14 @@
 			<artifactId>spring-cloud-starter</artifactId>
 		</dependency>
 		<dependency>
-			<!-- TODO: maybe this should be optional -->
 			<groupId>org.springframework.boot</groupId>
 			<artifactId>spring-boot-starter-web</artifactId>
+			<scope>optional</scope>
 		</dependency>
 		<dependency>
-			<!-- TODO: maybe this should be optional -->
 			<groupId>org.springframework.boot</groupId>
 			<artifactId>spring-boot-starter-actuator</artifactId>
+			<scope>optional</scope>
 		</dependency>
 		<dependency>
 			<groupId>org.springframework.boot</groupId>