Exclude commons-logging from starter POMs

Update all relevant starter POMs to include a `spring-core` dependency
with an exclusion on `commons-logging`. This prevents `commons-logging`
and `jcl-over-slf4j` from both being on the classpath.

Also add enforcer rules to ensure that commons-logging doesn't sneak
back in, and that there is no dependency convergence. (some additional
libraries were required in spring-boot-dependencies)

Tested with a sample maven project as well as using the `spring jar`
command.

Fixes gh-985

spring-boot-starters/spring-boot-starter-remote-shell/pom.xml

@@ -22,6 +22,16 @@
 			<groupId>org.springframework.boot</groupId>
 			<artifactId>spring-boot-starter-actuator</artifactId>
 		</dependency>
+		<dependency>
+			<groupId>org.springframework</groupId>
+			<artifactId>spring-core</artifactId>
+			<exclusions>
+				<exclusion>
+					<groupId>commons-logging</groupId>
+					<artifactId>commons-logging</artifactId>
+				</exclusion>
+			</exclusions>
+		</dependency>
 		<dependency>
 			<groupId>org.springframework</groupId>
 			<artifactId>spring-context</artifactId>