To suppress the warning

WARNING: no supported rendering engine found. To use Padrino::Helpers and
Padrino::Rendering properly you should include `gem 'erubis'`, `gem 'haml'`
or `gem 'slim'` in your Gemfile. If you are confident about using
Padrino::Helpers without Padrino::Rendering, please define constant
`Padrino::IGNORE_NO_RENDERING_ENGINE = true` before `require 'padrino-helpers'`.
use coveralls when COVERAGE is enabled