Add checkstyle plugin configuration

Not enabled by default because many projects do not use it yet. The
plan is to roll it out and then remove the google import detector.

Fixes gh-58

spring-cloud-build-tools/src/main/resources/checkstyle.xml

@@ -16,5 +16,8 @@
 		</module>
 		<module name="UnusedImports"/>
 		<module name="RedundantImport"/>
+		<module name="com.puppycrawl.tools.checkstyle.checks.imports.IllegalImportCheck" >
+			<property name="illegalPkgs" value="com.google.common"/>
+		</module>
 	</module>
-</module>
+</module>