Add ruby-dev as a dependency on Ubuntu

[davejagoda]

On Ubuntu 18.04, you'll need ruby-dev to be installed in order to build native extensions.

Description

README.md describes Ubuntu prerequisites, but it's missing one.

Checklist:

• I am associating a language with a new file extension.

  • The new extension is used in hundreds of repositories on GitHub.com
  • I have included a real-world usage sample for all extensions added in this PR
  • I have included a change to the heuristics to distinguish my language from others using the same extension.

• I am adding a new language.

  • The extension of the new language is used in hundreds of repositories on GitHub.com.
  • I have included a real-world usage sample for all extensions added in this PR
  • I have included a syntax highlighting grammar: https://github-lightshow.herokuapp.com/
  • I have updated the heuristics to distinguish my language from others using the same extension.

• I am fixing a misclassified language

  • I have included a new sample for the misclassified language
  • I have included a change to the heuristics to distinguish my language from others using the same extension.

• I am changing the source of a syntax highlighting grammar

• I am updating a grammar submodule

• I am adding new or changing current functionality

  • I have added or updated the tests for the new or changed functionality.

[pchaigno]

Thanks!