Fix: super_key should be set to CTRL on Linux
Fix: super_key should be set to CTRL on Linux

Since 983cf67 Linux key map has used CTRL instead of SUPER, but
common.util.super_key was still set to SUPER on Linux as
os.name == 'posix' is true both Linux and OS X. Condition changed
to sys.platform == 'darwin' which is only true on OS X.

Closes #190.

dvcrn/GitSavvy,divmain/GitSavvy,divmain/GitSavvy,asfaltboy/GitSavvy,theiviaxx/GitSavvy,jmanuel1/GitSavvy,stoivo/GitSavvy,ddevlin/GitSavvy,dvcrn/GitSavvy,jmanuel1/GitSavvy,theiviaxx/GitSavvy,stoivo/GitSavvy,dreki/GitSavvy,stoivo/GitSavvy,ralic/GitSavvy,ddevlin/GitSavvy,asfaltboy/GitSavvy,ddevlin/GitSavvy,divmain/GitSavvy,asfaltboy/GitSavvy,dreki/GitSavvy,ralic/GitSavvy