Fix gemspec files when is vendorized with bundler

When the gem is vendorized

for example:
s.files = ["/home/csaura/projects/my-project/vendor/cache/mail_view-35ca575d6fa4/init.rb", "/home/csaura/projects/my-project/vendor/cache/mail_view-35ca575d6fa4/Rakefile", "/home/csaura/projects/my-project/vendor/cache/mail_view-35ca575d6fa4/Gemfile", "/home/csaura/projects/my-project/vendor/cache/mail_view-35ca575d6fa4/Gemfile.lock", "/home/csaura/projects/my-project/vendor/cache/mail_view-35ca575d6fa4/README.md", "/home/csaura/projects/my-project/vendor/cache/mail_view-35ca575d6fa4/LICENSE", "/home/csaura/projects/my-project/vendor/cache/mail_view-35ca575d6fa4/lib", "/home/csaura/projects/my-project/vendor/cache/mail_view-35ca575d6fa4/lib/mail_view.rb", "/home/csaura/projects/my-project/vendor/cache/mail_view-35ca575d6fa4/lib/mail_view", "/home/csaura/projects/my-project/vendor/cache/mail_view-35ca575d6fa4/lib/mail_view/email.html.erb", "/home/csaura/projects/my-project/vendor/cache/mail_view-35ca575d6fa4/lib/mail_view/index.html.erb", "/home/csaura/projects/my-project/vendor/cache/mail_view-35ca575d6fa4/lib/mail_view/mapper.rb", "/home/csaura/projects/my-project/vendor/cache/mail_view-35ca575d6fa4/test", "/home/csaura/projects/my-project/vendor/cache/mail_view-35ca575d6fa4/test/test_mail_view.rb", "/home/csaura/projects/my-project/vendor/cache/mail_view-35ca575d6fa4/mail_view.gemspec"]