Use Ruby 2.0 caller_locations instead of caller if available

* we no more have to manipulate the each caller strings by ourselves using caller_locations

* caller_locations runs slightly faster, and creates less objects than good old caller
Benchmark (loading an Engine 1000 times):
  caller: 262.89 ms
  caller_locations: 186.068 ms

railties/lib/rails/engine.rb

@@ -351,8 +351,13 @@ module Rails
           Rails::Railtie::Configuration.eager_load_namespaces << base
 
           base.called_from = begin
-            # Remove the line number from backtraces making sure we don't leave anything behind
-            call_stack = caller.map { |p| p.sub(/:\d+.*/, '') }
+            call_stack = if Kernel.respond_to?(:caller_locations)
+              caller_locations.map(&:path)
+            else
+              # Remove the line number from backtraces making sure we don't leave anything behind
+              caller.map { |p| p.sub(/:\d+.*/, '') }
+            end
+
             File.dirname(call_stack.detect { |p| p !~ %r[railties[\w.-]*/lib/rails|rack[\w.-]*/lib/rack] })
           end
         end