EPUB guides improvements

[milmazz]

This PR includes the following changes:

• Replace named variables (e.g. assign on Liquid templates) for Elixir/Erlang versions.
• Rebuild yarn.lock to avoid problems with Gulp (see New release is buggy with Gulp ^3.9.0 nodejs/node#20285)
• Add basic styles for inline code, rebuilt assets after that.

As a side note, I checked the guides against epubcheck and everything appears to be working correctly:

~/D/e/e/_epub *master λ java -jar ~/Downloads/epubcheck-4.1.1/epubcheck.jar elixir-getting-started-guide.epub
Validating using EPUB version 3.0.1 rules.
No errors or warnings detected.
Messages: 0 fatal / 0 errors / 0 warnings / 0 info

EPUBCheck completed
~/D/e/e/_epub *master λ java -jar ~/Downloads/epubcheck-4.1.1/epubcheck.jar mix-and-otp.epub
Validating using EPUB version 3.0.1 rules.
No errors or warnings detected.
Messages: 0 fatal / 0 errors / 0 warnings / 0 info

EPUBCheck completed
~/D/e/e/_epub *master λ java -jar ~/Downloads/epubcheck-4.1.1/epubcheck.jar meta-programming-in-elixir.epub
Validating using EPUB version 3.0.1 rules.
No errors or warnings detected.
Messages: 0 fatal / 0 errors / 0 warnings / 0 info

[josevalim]

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