Change default markdown parser used by Jekyll.

The default maruku parser has an issue with nested lists;
see https://github.com/bhollis/maruku/issues/64 for details.
Using the rdiscount parser instead worked for the Oxford DTC boot camp.
Change markdown parser to redcarpet as suggested by Aron.
Merge pull request #98 from jonc125/nested-lists-fix

Work around a nested list formatting bug in the default markdown parser