Try to fix travis errors

See

* https://travis-ci.org/github/phstc/shoryuken/jobs/695504212
* pry/pry#2121 (comment)

shoryuken.gemspec

@@ -18,7 +18,7 @@ Gem::Specification.new do |spec|
   spec.require_paths = ['lib']
 
   spec.add_development_dependency 'dotenv'
-  spec.add_development_dependency 'pry-byebug'
+  spec.add_development_dependency 'pry-byebug', '3.9.0'
   spec.add_development_dependency 'rake'
   spec.add_development_dependency 'rspec'