Does not work if the current window is not opened as a folder

[zhiayang]

This is related to #15; if the current file is not opened as part of a folder (ie. it's just the file itself), then coq fails to start. I get this backtrace:

Traceback (most recent call last):
  File "/Applications/Sublime Text.app/Contents/MacOS/sublime_plugin.py", line 1088, in run_
    return self.run(edit)
  File "/Users/zhiayang/Library/Application Support/Sublime Text 3/Installed Packages/Coq.sublime-package/__init__.py", line 207, in run
  File "/Users/zhiayang/Library/Application Support/Sublime Text 3/Installed Packages/Coq.sublime-package/__init__.py", line 46, in start
IndexError: list index out of range

Using ST3 3.2.2, build 3211, the current up-to-date build on PackageControl.