PERF: ensure we run full GC on contexts

Prior to this change we would never clear memory from contexts and
rely on V8 reacting to pressure

This could lead to bloating of PrettyText and Transpiler contexts

This optimisations ensures that we will clear memory 2 seconds after
the last eval on the context

Gemfile.lock

@@ -188,7 +188,7 @@ GEM
     method_source (1.0.0)
     mini_mime (1.0.2)
     mini_portile2 (2.4.0)
-    mini_racer (0.2.11)
+    mini_racer (0.2.14)
       libv8 (> 7.3)
     mini_scheduler (0.12.2)
       sidekiq

lib/discourse_js_processor.rb

@@ -79,7 +79,7 @@ class DiscourseJsProcessor
 
     def self.create_new_context
       # timeout any eval that takes longer than 15 seconds
-      ctx = MiniRacer::Context.new(timeout: 15000)
+      ctx = MiniRacer::Context.new(timeout: 15000, ensure_gc_after_idle: 2000)
       ctx.eval("var self = this; #{File.read("#{Rails.root}/vendor/assets/javascripts/babel.js")}")
       ctx.eval(File.read(Ember::Source.bundled_path_for('ember-template-compiler.js')))
       ctx.eval("module = {}; exports = {};")

lib/js_locale_helper.rb

@@ -282,7 +282,7 @@ module JsLocaleHelper
   def self.with_context
     @mutex.synchronize do
       yield @ctx ||= begin
-        ctx = MiniRacer::Context.new(timeout: 15000)
+        ctx = MiniRacer::Context.new(timeout: 15000, ensure_gc_after_idle: 2000)
         ctx.load("#{Rails.root}/lib/javascripts/messageformat.js")
         ctx
       end

lib/pretty_text.rb

@@ -65,7 +65,7 @@ module PrettyText
   end
 
   def self.create_es6_context
-    ctx = MiniRacer::Context.new(timeout: 25000)
+    ctx = MiniRacer::Context.new(timeout: 25000, ensure_gc_after_idle: 2000)
 
     ctx.eval("window = {}; window.devicePixelRatio = 2;") # hack to make code think stuff is retina