Add coveralls support properly

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Gemfile

 source "http://rubygems.org"
 
-group :development do
+group :development, :test do
   gem 'coveralls', require: false
   gem 'rspec'
   gem 'webmock'

spec/spec_helper.rb

 ROOT_PATH = File.expand_path(File.join(File.dirname(__FILE__), ".."))
 
-if ENV['TRAVIS']
+if ENV['COVERALLS']
   require 'coveralls'
   Coveralls.wear!
 else