Add minor improvements to dev environment

- Git ignore tmp cache files
- Add pry-byebug to dev dependencies

Author: malcolmbaig

Diff for: .gitignore

@@ -13,6 +13,7 @@
 ## Dummy app temp files:
 /spec/dummy/db
 /spec/dummy/log
+/spec/dummy/tmp
 
 ## Specific to RubyMotion:
 .dat*

Diff for: jsonapi-rails.gemspec

@@ -23,4 +23,5 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency 'rspec-rails', '~> 3.5'
   spec.add_development_dependency 'with_model',  '~> 2.0'
   spec.add_development_dependency 'simplecov'
+  spec.add_development_dependency 'pry-byebug'
 end