Add libidn11-dev to Ubuntu package dependencies (#356)

This is now required as of Mastodon v1.5.0+ due to idn-ruby.

Running-Mastodon/Production-guide.md

@@ -88,7 +88,7 @@ Now you need to install [Yarn](https://yarnpkg.com/en/) plus some more software.
 - Other -dev packages, g++ - these are needed for the compilation of Ruby using ruby-build.
 
 ```sh
-apt -y install imagemagick ffmpeg libpq-dev libxml2-dev libxslt1-dev file git g++ libprotobuf-dev protobuf-compiler pkg-config nodejs gcc-6 autoconf bison build-essential libssl-dev libyaml-dev libreadline6-dev zlib1g-dev libncurses5-dev libffi-dev libgdbm3 libgdbm-dev nginx redis-server redis-tools postgresql postgresql-contrib nginx letsencrypt yarn
+apt -y install imagemagick ffmpeg libpq-dev libxml2-dev libxslt1-dev file git g++ libprotobuf-dev protobuf-compiler pkg-config nodejs gcc-6 autoconf bison build-essential libssl-dev libyaml-dev libreadline6-dev zlib1g-dev libncurses5-dev libffi-dev libgdbm3 libgdbm-dev nginx redis-server redis-tools postgresql postgresql-contrib nginx letsencrypt yarn libidn11-dev
 ```
 
 ### Dependencies That Need To Be Added As A Non-Root User