Calculate Bullet.app_root and IS_RUBY_19(former ruby_19?) once

Gem::Version.new and Gem::Version#<=> took more than half of all the
execution time of Bullet::Detector::NPlusOneQuery.call_association, and
Bullet.app_root roughly 1/5 more. None of this is changed without the
app restart, so it's safe to calculate them once.

lib/bullet.rb

@@ -73,7 +73,7 @@ def enable?
     end
 
     def app_root
-      (defined?(::Rails.root) ? Rails.root.to_s : Dir.pwd).to_s
+      @app_root ||= (defined?(::Rails.root) ? Rails.root.to_s : Dir.pwd).to_s
     end
 
     def n_plus_one_query_enable?

lib/bullet/stack_trace_filter.rb

@@ -3,6 +3,7 @@
 module Bullet
   module StackTraceFilter
     VENDOR_PATH = '/vendor'
+    IS_RUBY_19 = Gem::Version.new(RUBY_VERSION) < Gem::Version.new('2.0.0')
 
     def caller_in_project
       vendor_root = Bullet.app_root + VENDOR_PATH
@@ -47,19 +48,15 @@ def pattern_matches?(location, pattern)
     end
 
     def location_as_path(location)
-      ruby_19? ? location : location.absolute_path.to_s
+      IS_RUBY_19 ? location : location.absolute_path.to_s
     end
 
     def select_caller_locations
-      if ruby_19?
+      if IS_RUBY_19
         caller.select { |caller_path| yield caller_path }
       else
         caller_locations.select { |location| yield location }
       end
     end
-
-    def ruby_19?
-      @ruby_19 ||= Gem::Version.new(RUBY_VERSION) < Gem::Version.new('2.0.0') 
-    end
   end
 end