Ignore Gradle-related files and directories

When switching back to 3.1.x from master, ignore renamed directories,
Gradle 'build' dirs, generated IDE metadata, etc.

You may wish to clean these files with

    $ git clean -dfx

Or do a dry-run beforehand with the '-n' flag:

    $ git clean -dfxn

.gitignore

@@ -16,3 +16,11 @@ org.springframework.spring-parent/.project
 org.springframework.test/test-output/
 target
 spring-build/lib/docbook
+
+# ignore files and directories related to gradle build
+/.classpath
+/.project
+/.gradle
+/build
+/spring-*
+!/spring-framework-reference