Call constructor so cwd is set

I get errors in the console if this is not present:

```
Uncaught TypeError: Cannot call method 'pop' of undefined linter.coffee:217
  Linter.computeRange linter.coffee:217
  Linter.createMessage linter.coffee:160
  (anonymous function) linter.coffee:130
  self.forEach /Users/dmnd/github/linter/node_modules/xregexp/xregexp-all.js:542
  Linter.processMessage linter.coffee:129
  (anonymous function) linter.coffee:106
  (anonymous function) /Applications/Atom.app/Contents/Resources/app/src/buffered-process.js:77
  onread net.js:509
```

Test Plan:

Reload Atom window and notice there are no longer errors