gems. mime-types. update to 2.4.3

noticeable changes:

https://github.com/halostatue/mime-types/blob/fbdde28897b51c9de928fcc82b96709a74f848d8/History.rdoc#243--2014-10-21

- 2.4.3 fixed Ruby 1.9.2. bug
- 2.4.2 performance issue
- 2.4.2 prophylactic security fix related to JSON
gems. rails_12factor. update to 0.0.3.

Just fixes to README.md
https://github.com/heroku/rails_12factor/commits/master