A subset of small but nice updates.

Summary:
- Add some more files to ignore.
- Tiny std update for Travis config.
- Updated readme with build status and contribution information etc.
- Spree i18n namespace.
- Removed translation key "change_payment_method" that was not found.
- Add skrill translation keys that was not found in spree_i18n English version (I use this one to look up locale keys http://www.localeapp.com/projects/4605).
- Add locale for Swedish.
- Updated gemspec a bit.
- Let it use spree '~> 2.0' like I seen @huoxito do for an other module to avoid need for 2-0-stable branch.
- Add estimated version for rspec-rails, don't think it should hang to loose.