fix .gemspec

Due to a circular dependency in the `.gemspec` (sexp_path >= 0), the latest release of the gem is not installable. Based on my testing this seems to be due to the `gemspec` line in `Gemfile`. But repeating our dependencies already specified in `Rakefile` in `Gemfile` doesn't seem like a clean solution.

After some research, I decided to follow how the jeweler project does it, which is to specify all dependencies in `Gemfile`,

The re-generated `.gemspec` looks ok.