fix Nitro.us setup

[H1D]

According to #7:

I've created a box in Nitrous and changed folder to ~/workspace/stepic-plugins/stepic_plugins. Then executed ./conf_nitrous.sh.
This has led to the following error message:

Could not open requirements file: [Errno 2] No such file or directory: 'requirements.txt'                                                                                                 
Storing debug log for failure in /home/action/.pip/pip.log

So I just copied requirements.txt from the parent folder and re-launched ./conf_nitrous.sh.

[psviderski]

According to the stepic-plugin documentation http://stepic-plugins.readthedocs.org/en/latest/for_impatient.html you don't have to change dir to ~/workspace/stepic-plugins/stepic_plugins is order to execute conf_nitrous.sh. Just run it from the home directory: workspace/stepic-plugins/stepic_plugins/conf_nitrous.sh. If this is not convenient we will fix it soon.