Add .travis.yml

Task 11730 - add support for Travis CI

## Changes
* Add `.travis.yml`
* Ignore `Gemfile.lock` since it causes troubles to run specs in Travis env against different Rails versions
* Remove 1.8 gems from Gemfile
* Add status bars to README
* Use `protected_attribute` in test environment for Rails 4.0 (it was extracted)

## RSpec

https://travis-ci.org/TMXCredit/themis