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 dropping that and then repeating our dependencies already specified in `Rakefile` in `Gemfile` doesn't seem like a clean solution (that was the point of `gemspec` in the first place).

After some research, I decided to follow how the [jeweler project](https://github.com/technicalpickles/jeweler) does it: specifying dependencies in `Gemfile` (their [`Gemfile`](https://github.com/technicalpickles/jeweler/blob/master/Gemfile), [`Rakefile`](https://github.com/technicalpickles/jeweler/blob/master/Rakefile)).

The re-generated `.gemspec` looks ok. I'm not sure how to replace the released gem's gemspec though, though I have seen a "pulled" note next to some versions of gems on rubygems.

PS. Apologies for the spurious `%q` replacements, it could be due to the recent update to jeweler that my system picked up.