Update README.md (gem not released)

Hi,

Since `pagseguro-oficial` is not a released gem on rubygems, README was wrong.

Thank you guys for this gem!
Merge pull request #2 from jnettome/master

Fixing README instructions
Add escrow end date. Closes #3.
Lib identification
Indentation
Merge branch 'master' of https://github.com/pagseguro/ruby
Update payment_request.rb
Update payment_request.rb
Update payment_request.rb
Update success.xml
Update spec to conform new Aitch API.
Update failure.xml
Merge branch 'desenvolvimento' of https://github.com/pagseguro/ruby into desenvolvimento
Update payment_request.rb
Update pt-BR.yml
Update gitignore
Merge branch 'master' into desenvolvimento
Update PaymentMethodType
Update README.md
Update pt-BR.yml