Add ruby-dev as an installation dependency

[oliveratutexas]

I think it would help to add the ruby-dev package as a dependency on the homepage. When I installed on 16.04 Ubuntu I needed that dependency.

[besseker]

Met the same problem on Ubuntu 16.04. Couldn't install using gem install bropages.
Used sudo apt-get install ruby-dev .
and it worked afterwards.

[OpnSrcConstruction]

I'm running 16.04 LTS as the base for all of my local OS's, and ruby-dev fixed it, with the exception of when I had accidentally installed/uninstalled bro through the package manager; which installed bro NT tool. After installing ruby-dev through apt w/ suoo apt install ruby-dev, and gem install bropages; I got an error couldn't find command(even though the which command found bro in a valid $PATH directory). I fixed the issue by doing the following symlink:
$ sudo ln -s /usr/local/bin/bro /usr/bin/bro

[gitcomrade]

For Ubuntu 17.10 and 18.04 beta (possibly other OSes) ruby-unf and ruby-unf-ext are also installation dependencies.