fix jekyll detection
fix jekyll detection

Summary:
upon upgrading to bundler 2, binaries aren't installed on the path anymore (without `bundle install --binstubs`), and you have to use `bundle exec` which modifies the env including `PATH`.

that was causing our Makefile to think that jekyll wasn't installed and abort.

Reviewed By: gkz

Differential Revision: D21751132

fbshipit-source-id: adbfd5f06b6741ff50cb6fe949d570c2178a9638

nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow