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