Fix errors when watch triggers on files which are already removed.
Added Ubuntu installation instructions
Modified to show executable commands in code blocks
Merge branch 'master' of https://github.com/bkockx/javascript-kata into bkockx-master
Put the Ubuntu section before the link section.

- Set the headings of the Ubuntu section one level deeper.
- Add extra whitespace such that the code blocks are actually rendered as
  such.