Updating gems

Due an old version of passenger, the bundle process would fail on ruby
2.0.0. Taking the opportunity to also update the remaining gems
Switch redcarpet to kramdown

Kramdown is a another markdown parser, fast and using pure Ruby. Beacuse
of this, it is possible to run gistub in a JRuby environment. Using
Redcarpet it was not possible, given it relies on a native C extension.
Removed assets logging in dev mode
Removed passenger by default, added better_errors
version 1.0.13
Merge branch 'release/1.0.13'