Add missing CommandInstaller require

Closes #3729
Upgrade to language-css@0.18
Upgrade to language-yaml@0.18
Upgrade to language-ruby@0.39
Add tabLength 3

Closes #3724
Upgrade to language-gfm@0.51
Register workspace commands via $::command until compatibility is better

This is a temporary measure.

Currently, commands registered via atom.commands.add don’t mesh properly
with the dispatch of commands registered via jQuery. Didn’t think this
was a big deal until I realized that it broke the ability to preempt
commands on ancestor nodes by calling stopPropagation, which might break
packages.
Move remaining workspace commands back to $::command temporarily
Upgrade to apm 0.101
Version bump Squirrel for Windows to 0.5.3
Download previous releases so that Squirrel will build updates for them
Kill trailing whitespace
We've got a valid ReleasesDir, don't clear it
Prepare 0.137
Attach, show and hide pane item views with native DOM APIs

We continue to invoke SpacePen attach hooks on any shim wrappers
for backward compatibility, but SpacePen wrappers are no longer required
for attoch/hide/show as they were previously. Next: removal.
Use native DOM APIs to remove pane item views
Upgrade to language-todo@0.13
Upgrade to language-css@0.19
:lipstick: Remove unnecessary conditional

`mkdir -p` is an idempotent operation.
Merge pull request #3750 from mlloreda/patch-1

:lipstick: Remove unnecessary conditional