add ruby version

With 1.9.3, you got "undefined method `caller_locations'" error. With 2.1, debugger gem isn't supported. So 2.0.0 it is.

Signed-off-by: Joshua Paling <joshua.paling@gmail.com>
change 'topics' nav to 'courses'

as per Tim's suggestion

Signed-off-by: Joshua Paling <joshua.paling@gmail.com>
add courses from Rails Girls Next

Signed-off-by: Joshua Paling <joshua.paling@gmail.com>