Add twisted-15.5.0 module to webkitpy.thirdparty.autoinstalled.

Add twisted-15.5.0 module to webkitpy.thirdparty.autoinstalled.
https://bugs.webkit.org/show_bug.cgi?id=154667

Reviewed by Ryosuke Niwa and Simon Fraser.

Add twisted-15.5.0 to webkitpy.thirdparty.autoinstalled.
Minor fix for twsited_http_server used by run-benchmark that we should only stop http server while recieving 'GET /shutdown'.

* Scripts/webkitpy/benchmark_runner/http_server_driver/http_server/twisted_http_server.py:
(ServerControl.render_POST): Deleted.
* Scripts/webkitpy/thirdparty/__init__.py:
(AutoinstallImportHook.find_module):
(AutoinstallImportHook._install_twisted_15_5_0):
* Scripts/webkitpy/thirdparty/__init___unittest.py:
(ThirdpartyTest.test_imports):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@198187 268f45cc-cd09-0410-ab3c-d52691b4dbfc