Only load rblineprof when actually needed

This ensures the application can still boot when the "development" group
is not available.

lib/gitlab/sherlock.rb

@@ -1,5 +1,4 @@
 require 'securerandom'
-require 'rblineprof' if RUBY_ENGINE == 'ruby'
 
 module Gitlab
   module Sherlock

lib/gitlab/sherlock/line_profiler.rb

@@ -40,6 +40,8 @@ module Gitlab
 
       # Profiles the given block using rblineprof (MRI only).
       def profile_mri
+        require 'rblineprof'
+
         retval  = nil
         samples = lineprof(/^#{Rails.root.to_s}/) { retval = yield }