Add libidn11 to Aptfile (#4475)

Fix builds that fail on Heroku.

Aptfile

@@ -1,8 +1,9 @@
+ffmpeg
+libicu-dev
+libidn11
+libidn11-dev
 libpq-dev
-protobuf-compiler
 libprotobuf-dev
-ffmpeg
 libxdamage1
 libxfixes3
-libicu-dev
-libidn11-dev
+protobuf-compiler