Backport 'check_output' to Python 2.6.x

- 'subprocess.check_output' is not supported in Sublime Text 2, as
  it uses an older version of Python (2.6.5). Backport this function
Remove unsupported 'encoding' argument

- 'decode' does not support the above argument in Python 2.6.x.
  Remove this as it's unecessary.