auto-load Gemfile (if present)

Many of the demo stasis projects I've seen include a Gemfile. This patches the `stasis` executable to auto-load the gems listed in the Gemfile into the project. It's in a rescue block, and if no Gemfile is present then no errors rise.

Currently, (I think) the standard approach would be to explicitly require haml, scss, etc in a top level controller, or include the two lines `require "bundler/setup"; Bundler.require` in a top level controller. This patch makes including those requires unnecessary.