Bump minimum required pry version to 0.10.4

Running pry-rails against versions of pry prior to version 0.10.4 will
cause an 'uninitialized constant' exception for Pry::Prompt. No class by
this name existed prior to version 0.10.4.

pry-rails.gemspec

@@ -20,7 +20,7 @@ Gem::Specification.new do |s|
   s.executables   = `git ls-files -- bin/*`.split("\n").map{ |f| File.basename(f) }
   s.require_paths = ["lib"]
 
-  s.add_dependency "pry", ">= 0.9.10"
+  s.add_dependency "pry", ">= 0.10.4"
   s.add_development_dependency "appraisal"
   s.add_development_dependency "minitest"
 end