Fix to UnicodeEncodeError at bin/make_bootcamp_rss_feed.py

@gvwilson discovery a issue with non ASCII characters at YAML headers when generating the bootcamp RSS feed since [r-gaia-cs/2014-05-12-furg](http://r-gaia-cs.github.io/2014-05-12-furg) has a `á` at the address field. Similar same problem would happen when Rémi Emonet or Julián García be a instructor.

I hope this fix the issue.

~~~
$  python --version
Python 2.7.6
$ make clean cache site
cp ./config/bootcamps_saved.yml ./_bootcamp_cache.yml
make SITE=/home/raniere/documents/software_carpentry/site/_site OUT=/home/raniere/documents/software_carpentry/site/_site build
make[1]: Entering directory '/home/raniere/documents/software_carpentry/site'
python ./bin/preprocess.py -c ./config -o /home/raniere/documents/software_carpentry/site/_site -s /home/raniere/documents/software_carpentry/site/_site
jekyll build -d /home/raniere/documents/software_carpentry/site/_site
Configuration file: /home/raniere/documents/software_carpentry/site/_config.yml
            Source: /home/raniere/documents/software_carpentry/site
       Destination: /home/raniere/documents/software_carpentry/site/_site
      Generating... done.
python ./bin/make_calendar.py -o /home/raniere/documents/software_carpentry/site/_site -s /home/raniere/documents/software_carpentry/site/_site
python ./bin/make_rss_feed.py -o /home/raniere/documents/software_carpentry/site/_site -s /home/raniere/documents/software_carpentry/site/_site
python ./bin/make_bootcamp_rss_feed.py -o /home/raniere/documents/software_carpentry/site/_site -s /home/raniere/documents/software_carpentry/site/_site
cp _htaccess /home/raniere/documents/software_carpentry/site/_site/.htaccess
make[1]: Leaving directory '/home/raniere/documents/software_carpentry/site'
~~~