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/pom.xml

@@ -53,4 +53,32 @@
 		<module>spring-boot-starter-web</module>
 		<module>spring-boot-starter-websocket</module>
 	</modules>
+	<build>
+		<plugins>
+			<plugin>
+				<groupId>org.apache.maven.plugins</groupId>
+				<artifactId>maven-enforcer-plugin</artifactId>
+				<executions>
+					<execution>
+						<id>enforce-rules</id>
+						<goals>
+							<goal>enforce</goal>
+						</goals>
+						<configuration>
+							<rules>
+								<bannedDependencies>
+									<excludes>
+										<exclude>commons-logging:commons-logging:*</exclude>
+									</excludes>
+									<searchTransitive>true</searchTransitive>
+								</bannedDependencies>
+								<dependencyConvergence/>
+							</rules>
+							<fail>true</fail>
+						</configuration>
+					</execution>
+				</executions>
+			</plugin>
+		</plugins>
+	</build>
 </project>