Add duration pytest option to pants.travis-ci.ini

This adds a pytest option for printing out the slowest running tests. I set it to the 10 slowest, but we could dial it back to 3 or 1. It looks like this:

                     ============== test session starts ===============

                     platform linux2 -- Python 2.7.3 -- py-1.4.31 -- pytest-2.6.4

                     plugins: timeout, cov

                     collected 3 items

                     contrib/node/tests/python/pants_test/contrib/node/tasks/test_node_resolve_integration.py ..

23:43:33 09:43         [chroot].

                     =========== slowest 10 test durations ============

                     95.48s call     contrib/node/tests/python/pants_test/contrib/node/tasks/test_node_resolve_integration.py::NodeResolveIntegrationTest::test_resolve_local_and_3rd_party_dependencies

                     18.03s call     contrib/node/tests/python/pants_test/contrib/node/tasks/test_node_resolve_integration.py::NodeResolveIntegrationTest::test_resolve_with_prepublish

                     1.89s call     contrib/node/tests/python/pants_test/contrib/node/tasks/test_node_resolve_integration.py::NodeResolveIntegrationTest::test_resolve_preinstalled_node_module_project

                     0.00s setup    contrib/node/tests/python/pants_test/contrib/node/tasks/test_node_resolve_integration.py::NodeResolveIntegrationTest::test_resolve_with_prepublish

                     0.00s teardown contrib/node/tests/python/pants_test/contrib/node/tasks/test_node_resolve_integration.py::NodeResolveIntegrationTest::test_resolve_preinstalled_node_module_project

                     0.00s teardown contrib/node/tests/python/pants_test/contrib/node/tasks/test_node_resolve_integration.py::NodeResolveIntegrationTest::test_resolve_local_and_3rd_party_dependencies

                     0.00s setup    contrib/node/tests/python/pants_test/contrib/node/tasks/test_node_resolve_integration.py::NodeResolveIntegrationTest::test_resolve_preinstalled_node_module_project

                     0.00s setup    contrib/node/tests/python/pants_test/contrib/node/tasks/test_node_resolve_integration.py::NodeResolveIntegrationTest::test_resolve_local_and_3rd_party_dependencies

                     0.00s teardown contrib/node/tests/python/pants_test/contrib/node/tasks/test_node_resolve_integration.py::NodeResolveIntegrationTest::test_resolve_with_prepublish

                     =========== 3 passed in 115.46 seconds ===========

-- https://travis-ci.org/pantsbuild/pants/jobs/121572193#L2669

Reviewed at https://rbcommons.com/s/twitter/r/3662/

pants.travis-ci.ini

@@ -9,3 +9,7 @@ use_nailgun: False
 # and get oomkilled from launching too many workers with too much total memory
 # overhead.
 worker_count: 4
+
+
+[test.pytest]
+options: ['--duration=3']