Do not alter pull_requests while iterating on it

Deleting elements of an array while iterating on it
results in never seeing the last element.

    > a = [1,2,3]
     => [1, 2, 3]
    > a.each do |e|
    >   p a
    >   a.delete(e)
    > end
    [1, 2, 3]
    [2, 3]
     => [2]

lib/github_changelog_generator/generator/generator.rb

@@ -120,22 +120,22 @@ module GitHubChangelogGenerator
         issues_a.push dict unless added
       end
 
+      added_pull_requests = []
       pull_requests.each do |dict|
-        added = false
         dict.labels.each do |label|
           if @options[:bug_labels].include? label.name
             bugs_a.push dict
-            added = true
+            added_pull_requests.push dict
             next
           end
           if @options[:enhancement_labels].include? label.name
             enhancement_a.push dict
-            added = true
+            added_pull_requests.push dict
             next
           end
         end
-        pull_requests.delete(dict) if added
       end
+      added_pull_requests.each { |p| pull_requests.delete(p) }
 
       [bugs_a, enhancement_a, issues_a]
     end