relax gem version
Add slack notification
Refactor build ruby versions
tweak badge
use coveralls when COVERAGE is enabled
Add releasenote
fix on ruby 2.0 error

/home/travis/.rvm/gems/ruby-2.0.0-p598/gems/activesupport-4.1.8/lib/active_support/number_helper.rb:3:in `<module:NumberHelper>': uninitialized constant ActiveSupport::Autoload (NameError)
  from /home/travis/.rvm/gems/ruby-2.0.0-p598/gems/activesupport-4.1.8/lib/active_support/number_helper.rb:2:in `<module:ActiveSupport>'
  from /home/travis/.rvm/gems/ruby-2.0.0-p598/gems/activesupport-4.1.8/lib/active_support/number_helper.rb:1:in `<top (required)>'
  from /home/travis/.rvm/gems/ruby-2.0.0-p598/gems/activesupport-4.1.8/lib/active_support/core_ext/numeric/conversions.rb:2:in `require'
  from /home/travis/.rvm/gems/ruby-2.0.0-p598/gems/activesupport-4.1.8/lib/active_support/core_ext/numeric/conversions.rb:2:in `<top (required)>'
Merge pull request #4 from sue445/feature/upgrade_gems

Feature/upgrade gems