Add frozen string literal comment to files.

Gemfile

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 source 'https://rubygems.org'
 
 group :development do

bin/pull_translations.rb

@@ -1,4 +1,5 @@
 #!/usr/bin/env ruby
+# frozen_string_literal: true
 
 # Usage:
 #   bundle install

lib/math_renderer.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # the idea here is to support server rendering
 # there is a node example and we could inject this in a post processing job
 class DiscourseMath::MathRenderer

plugin.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # name: discourse-math
 # about: Official mathjax plugin for Discourse
 # version: 0.9

spec/lib/math_renderer_spec.rb

@@ -1 +1,3 @@
+# frozen_string_literal: true
+
 # TODO server rendering

spec/pretty_text_spec.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'rails_helper'
 
 describe PrettyText do