Load gems with bundler in script/github_pr_errors

It makes running the script more deterministic

It also prevents warnings like `WARN: Unresolved or ambiguous specs during Gem::Specification.reset` to be displayed when there are multiple versions of the same gem in the load path.

[skip ci]

script/github_pr_errors

@@ -1,5 +1,9 @@
 #!/usr/bin/env ruby
 
+require 'rubygems'
+require 'bundler'
+Bundler.setup(:default, :development)
+
 require 'pathname'
 require 'json'
 require 'rest-client'