Fixed for install_theme error:  "TypeError: unicode argument expected" 

Hi.  With Nikola 5.1, I got this error when attempting to use the `install_theme` command:

    $ nikola install_theme -n blogtxt
    Downloading: http://nikola.ralsina.com.ar/themes/blogtxt.zip
    Traceback (most recent call last):
      File "/Library/Frameworks/Python.framework/Versions/Current/bin/nikola", line 66, in <module>
        site.commands[cmd_name].run(*sys.argv[2:])
      File "/Library/Frameworks/Python.framework/Versions/Current/lib/python2.7/site-packages/nikola/plugins/command_install_theme.py", line 88, in run
        zip_file.write(requests.get(data[name]).content)
    TypeError: unicode argument expected, got 'str'

This was on Mac OS X 10.8 with Python 2.7.3.  I built Nikola from source with the master branch, and reproduced the error.

Here's a pull request which fixed the error for me.  Thanks!