Added the environment variable HUBOT_SKYPE_ADAPTER_PYTHON for a custom location of the python executable

Signed-off-by: Philipp Beckmann <philippbeckmann@pabnet.de>
Update src/skype.coffee

Renamed env var to HUBOT_SKYPE_PYTHON to match existing vars
Merge pull request #14 from philippbeckmann/Branch_envvar

Environment variable for custom location of the python executable