Added ./index.html as dependency for INDEX

Not quite sure if actually not fixing something that ain't broken but when preparing a boocamp page, I often change 'index.htm'. In order to see if it's OK, I run 'make site' but since ./index.html is not a dependency for 'site' (in Makefile in gh-pages) nothing happens. In the 'master' branch such dependency exists. But the guidelines for creating new bootcamp repositories specify that instructors copy the contents of 'gh-pages'. So I guess it's the Makefile in gh-pages that should include the dependency too?