Add configuration necessary for coveralls reporting

NOTE: in addition to the code changes here, the Coveralls API Key
needed to be added to the environment variables in the CircleCI
Project settings here:
https://circleci.com/gh/samvera-labs/iiif_manifest/edit#env-vars

The Coveralls API Key should not be checked into any public repository.
See additional docs here http://docs.coveralls.io/ruby-on-rails

.coveralls.yml

@@ -0,0 +1 @@
+service_name: travis-ci

iiif_manifest.gemspec

@@ -20,7 +20,7 @@ Gem::Specification.new do |spec|
   spec.add_dependency 'activesupport', '>= 4'
 
   spec.add_development_dependency 'bixby', '~> 1.0'
-  spec.add_development_dependency 'bundler', '~> 1.11'
+  spec.add_development_dependency 'coveralls'
   spec.add_development_dependency 'pry-byebug'
   spec.add_development_dependency 'rake', '~> 10.0'
   spec.add_development_dependency 'rspec', '~> 3.0'

spec/spec_helper.rb

@@ -1,3 +1,6 @@
+require 'coveralls'
+Coveralls.wear!
+
 $LOAD_PATH.unshift File.expand_path('../lib', __dir__)
 require 'iiif_manifest'
 require 'pry'