Correct contributing part of README according to travis config

As I can see we _always_ need to use dummy app for testing.
Merge pull request #4 from freemanoid/patch-1

Correct contributing part of README according to travis config
Add boot.rb with custom Gemfile path
Merge pull request #5 from freemanoid/default-gemfile-for-dummy

Fix default gemfile for dummy application
renamed gem to pludoni-simple_captcha for now
renamed to simple_captcha2
drop Ruby 1.8.7
converted README to markdown with syntax highlighting
deleted old README, fixed markdown formatting issues
...
clarification: needs to require simple_captcha for now
test usage
Update README.md
gemspec warning fixed; travis.yml now with ruby 2.1 and rails 4.1beta
tests now work
v.0.2.1: fixed bundle warning, missing README.rdoc
maybe fix: uninitialized constant