Update sxp repo location in Gemfile.

Author: gkellogg

Gemfile

@@ -7,7 +7,7 @@ gem 'jsonlint',         github: "dougbarth/jsonlint",       platforms: [:rbx, :m
 
 group :development do
   gem 'ebnf',           github: "gkellogg/ebnf",            branch: "develop"
-  gem 'sxp',            github: "gkellogg/sxp-ruby",        branch: "develop"
+  gem 'sxp',            github: "dryruby/sxp.rb",           branch: "develop"
   gem 'rdf-isomorphic', github: "ruby-rdf/rdf-isomorphic",  branch: "develop"
   gem 'rdf-turtle',     github: "ruby-rdf/rdf-turtle",      branch: "develop"
   gem 'rdf-trig',       github: "ruby-rdf/rdf-trig",        branch: "develop"