Bump rack dependency to 1.4.3

It includes security bug fixes and changes the initialization of
Rack::File to accept a hash, otherwise generating warnings.

See 295806e1 for the warnings fix.

actionpack/actionpack.gemspec

@@ -21,7 +21,7 @@
 
   s.add_dependency 'activesupport', version
   s.add_dependency 'builder',       '~> 3.1.0'
-  s.add_dependency 'rack',          '~> 1.4.1'
+  s.add_dependency 'rack',          '~> 1.4.3'
   s.add_dependency 'rack-test',     '~> 0.6.1'
   s.add_dependency 'erubis',        '~> 2.7.0'