Add *.iml files to gitignore

Android Studio has a tendency to create .iml files for gradle projects.
Add this file type to .gitignore.

Change-Id: Id5fcae50f267a5d60d1d4c0c69417d95381cfc02
diff --git a/.gitignore b/.gitignore
index ba077a4..4471aab 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1 +1,2 @@
 bin
+*.iml