Check that cli jar command only writes .jars

Update `JarCommand` to check that the file extension of the output is
`.jar`.

Fixes gh-581

spring-boot-cli/src/main/java/org/springframework/boot/cli/command/jar/JarCommand.java

@@ -110,6 +110,8 @@ public class JarCommand extends OptionParsingCommand {
 					"The name of the resulting jar and at least one source file must be specified");
 
 			File output = new File((String) nonOptionArguments.remove(0));
+			Assert.isTrue(output.getName().toLowerCase().endsWith(".jar"), "The output '"
+					+ output + "' is not a JAR file.");
 			deleteIfExists(output);
 
 			GroovyCompiler compiler = createCompiler(options);