Set cwd when executing Popen in Linux

I noticed that cwd was not set in Linux, so none of the K commands were executed in Sublime Text 3, since those could not find project.json. After setting cwd, all the K commands could be executed normally.

I was also tempted to remove the "else if", since after my change, it does the same as the "else" part, but I thought that maybe you might want to add other functionalities for linux in the future.