[Minor]ignore .idea_modules

ignore .idea_modules ,  ```sbt/sbt gen-idea``` generate this dir.

Author: wangfei <[email protected]>

Closes apache#2476 from scwf/patch-4 and squashes the following commits:

e6ab88a [wangfei] ignore .idea_modules

Author: scwf

.gitignore

@@ -6,6 +6,7 @@
 *.iml
 *.iws
 .idea/
+.idea_modules/
 sbt/*.jar
 .settings
 .cache