Update dependencies, RSpec, etc.

Hi Didier,

how are you, hope you had a merry Christmas.

Because I need `custom_fields` next year for a project, I thought to myself, maybe i could update it.

All work is based on the `mongoid4` branch. Hopefully, the commit is not too large.

What i did:

* Updated all dependencies, except `rails` and `money`
* Updated RSpec to version 3.1
* Improved some specs
* Updated README and Gemspec
* Removed `# encoding: utf-8` comments
* Updated Travis CI configuration

Could you please look at the changes?

Wish you a happy new year!

Maik