Require single main class

Update run tasks to ensure that only a single main class is required
when performing a class search.

See gh-886

spring-boot-tools/spring-boot-maven-plugin/src/main/java/org/springframework/boot/maven/RunMojo.java

@@ -217,7 +217,7 @@ public class RunMojo extends AbstractDependencyFilterMojo {
 		String mainClass = this.mainClass;
 		if (mainClass == null) {
 			try {
-				mainClass = MainClassFinder.findMainClass(this.classesDirectory);
+				mainClass = MainClassFinder.findSingleMainClass(this.classesDirectory);
 			}
 			catch (IOException ex) {
 				throw new MojoExecutionException(ex.getMessage(), ex);