Add autoprefixer-rails gem

https://github.com/ai/autoprefixer-rails
fix #331 cardのvalidation
Merge pull request #422 from mozilla-japan/task331

fix #331 cardのvalidation
fix #421  jquery parsererror when creating a card with image file
Merge pull request #423 from mozilla-japan/bug421

fix #421  jquery parsererror when creating a card with image file
Merge pull request #420 from shinnn/autoprefixer-rails

fix #418 add autoprefixer-rails gem
fix #363 About page::create html
Merge pull request #424 from mozilla-japan/task363

fix #363 About page::create html
close #359 403::create html&css
close #362 503::create html&css
Merge pull request #425 from mozilla-japan/task359

close #359 403::create html&css
Merge pull request #426 from mozilla-japan/task362

close #362 503::create html&css
close #357 401::create css
close #364 Terms and Conditions:: create html
Merge pull request #427 from mozilla-japan/task357

close #357 401::create css
Merge pull request #428 from mozilla-japan/task364

close #364 Terms and Conditions:: create html