[bug] gu (Atom) - check for apm (#19)

Fix bug where user previously installed Atom, which created `~/.atom`, but then uninstalled Atom - which leaves `~/.atom` behind.
* The script shows an error that `apm` command doesn't exist, so it should check to make sure the command exists.
* The script should still check for `.atom` given that the script also relies on that folder being present.

bin/gu

@@ -200,7 +200,7 @@ else
  fi
 
  ### ATOM
- if [ -d .atom ]; then
+ if [ -x "$(command -v apm)" ] && [ -d .atom ]; then
  u 'apm' 'apm list --installed --no-color'
  # TODO: 'font size'/'show panel' change shouldn't trigger update
  # always back up dark theme so that a theme change doesn't trigger a gu update