Change to use Rainbow instead of colorize

colorize is licensed under GPL-2 which conflicts with the MIT license of
github_changelog_generator.  This changes all usage of colorize to
rainbow which does have a compatible license (MIT)

lib/github_changelog_generator/generator/generator_fetcher.rb

@@ -74,7 +74,7 @@ module GitHubChangelogGenerator
           commit = @fetcher.fetch_commit(event)
           issue[:actual_date] = commit[:author][:date]
         rescue
-          puts "Warning: Can't fetch commit #{event[:commit_id]}. It is probably referenced from another repo.".yellow
+          puts "Warning: Can't fetch commit #{event[:commit_id]}. It is probably referenced from another repo.".color(:yellow)
           issue[:actual_date] = issue[:closed_at]
         end
       end

lib/github_changelog_generator/generator/generator_generation.rb

@@ -39,10 +39,10 @@ module GitHubChangelogGenerator
           index2 = hash[tag2]
           log += generate_log_between_tags(all_tags[index1], all_tags[index2])
         else
-          raise ChangelogGeneratorError, "Can't find tag #{tag2} -> exit".red
+          raise ChangelogGeneratorError, "Can't find tag #{tag2} -> exit".color(:red)
         end
       else
-        raise ChangelogGeneratorError, "Can't find tag #{tag1} -> exit".red
+        raise ChangelogGeneratorError, "Can't find tag #{tag1} -> exit".color(:red)
       end
       log
     end

lib/github_changelog_generator/generator/generator_tags.rb

@@ -19,7 +19,7 @@ module GitHubChangelogGenerator
     # @param [Hash] tag_name name of the tag
     # @return [Time] time of specified tag
     def get_time_of_tag(tag_name)
-      raise ChangelogGeneratorError, "tag_name is nil".red if tag_name.nil?
+      raise ChangelogGeneratorError, "tag_name is nil".color(:red) if tag_name.nil?
 
       name_of_tag = tag_name["name"]
       time_for_name = @tag_times_hash[name_of_tag]