Use project root as cwd

I have a pylint file in my project root that wasn't being found by
`linter-pylint`. Looking into it, I discovered `@cwd` was `null` because
`LinterPylint.constructor` never calls `super`.

Test Plan:
Opened a new Atom window in the directory that contains my pylint file and
had pylint import things properly.
use existence operator
Merge pull request #8 from dmnd/project-root

Use project root as cwd