Dutch translation updated

Up til commit dd400bb
Refactored post model to reduce duplication in validator methods
Require rspec-rails in Gemfile to fix 'spec' task
Merge pull request #857 from chrishunt/chrishunt/fix-spec-rake-task

Require rspec-rails in Gemfile to fix 'spec' task
Merge pull request #850 from bplexico/refactor_post_rb

Refactored post model to reduce duplication in validator methods
Merge pull request #855 from sanderdatema/dutch_translation

Dutch translation updated