deps: declare parselt — the ISC parser's runtime dependency

The parser requires parselt but it was never declared; it only worked
where parselt happened to be installed. Declared in the gemspec
(~> 2.0, no transitive deps).

Author: ronaldtse

interscript.gemspec

@@ -35,4 +35,5 @@ Gem::Specification.new do |spec|
   spec.add_dependency "thor"
   spec.add_dependency "interscript-maps", "~> #{Interscript::VERSION.split(".")[0, 2].join(".")}.0a"
   spec.add_dependency "text"
+  spec.add_dependency "parselt", "~> 2.0"
 end