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

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