Get environment variable w/o killing CPU on linux

[kbrose]

This is an attempt to fix #20.

I do not have access to a Mac, so I cannot check verify that this change does't re-introduce #4. Although, hacking Travis CI on a different atom package seemed to indicate that process.env contained the correct information on the mac environment, but Travis CI also starts Atom from a shell.

Ref:

• Add atom.whenShellEnvironmentLoaded() atom/atom#14164
• Make updateProcessEnv async and add core:loaded-shell-environment activation hook atom/atom#13200
• Get environment variable w/o killing CPU on linux #27

[kbrose]

#29 seems to indicate this approach won't work either :(