Fix straggler usage of legacy psutil form

- Fix straggler usage of legacy psutil property form in pidlock.py.
- Add a simple test case to exercise the psutil dependency to catch this in CI in the future.

Testing Done:
https://travis-ci.org/pantsbuild/pants/builds/73467865

with the new test, pre-fix:

[illuminati pants (kwlzn/psutil_fixup)]$ PANTS_DEV=1 ./pants test tests/python/pants_test/process::
...
15:36:02 00:00     [pytest]
15:36:02 00:00       [run]
                     ============== test session starts ===============
                     platform darwin -- Python 2.7.6 -- py-1.4.30 -- pytest-2.6.4
                     plugins: timeout, cov
                     collected 9 items

                     tests/python/pants_test/process/test_pidlock.py F
                     tests/python/pants_test/process/test_xargs.py ........

                     ==================== FAILURES ====================
                      TestOwnerPrintingPIDLockFile.test_cmdline_for_pid

                     self = <pants_test.process.test_pidlock.TestOwnerPrintingPIDLockFile testMethod=test_cmdline_for_pid>

                         def test_cmdline_for_pid(self):
                     >     self.assertIsInstance(self.obj.cmdline_for_pid(os.getpid()))

                     tests/python/pants_test/process/test_pidlock.py:19:
                     _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

                     pid = 82357

                         @staticmethod
                         def cmdline_for_pid(pid):
                           try:
                             process = psutil.Process(pid)
                     >       return ' '.join(process.cmdline)
                     E       TypeError

                     src/python/pants/process/pidlock.py:38: TypeError
                     ======= 1 failed, 8 passed in 0.05 seconds =======
...
FAILURE

and with the fix:

[illuminati pants (kwlzn/psutil_fixup)]$ PANTS_DEV=1 ./pants test tests/python/pants_test/process::
...
15:38:41 00:00     [pytest]
15:38:41 00:00       [run]
                     ============== test session starts ===============
                     platform darwin -- Python 2.7.6 -- py-1.4.30 -- pytest-2.6.4
                     plugins: timeout, cov
                     collected 9 items

                     tests/python/pants_test/process/test_pidlock.py .
                     tests/python/pants_test/process/test_xargs.py ........

                     ============ 9 passed in 0.04 seconds ============
...
               SUCCESS

Bugs closed: 1889

Reviewed at https://rbcommons.com/s/twitter/r/2546/

src/python/pants/process/pidlock.py

@@ -35,6 +35,6 @@ def acquire(self, timeout=None):
   def cmdline_for_pid(pid):
     try:
       process = psutil.Process(pid)
-      return ' '.join(process.cmdline)
+      return ' '.join(process.cmdline())
     except psutil.NoSuchProcess:
       return None

tests/python/pants_test/process/BUILD

@@ -6,6 +6,7 @@ python_tests(
   sources = globs('*.py'),
   dependencies = [
     '3rdparty/python:mox',
+    '3rdparty/python:six',
     'src/python/pants/process',
   ]
 )

tests/python/pants_test/process/test_pidlock.py

@@ -0,0 +1,21 @@
+# coding=utf-8
+# Copyright 2015 Pants project contributors (see CONTRIBUTORS.md).
+# Licensed under the Apache License, Version 2.0 (see LICENSE).
+
+from __future__ import (absolute_import, division, generators, nested_scopes, print_function,
+                        unicode_literals, with_statement)
+
+import os
+import unittest
+
+import six
+
+from pants.process.pidlock import OwnerPrintingPIDLockFile
+
+
+class TestOwnerPrintingPIDLockFile(unittest.TestCase):
+  def setUp(self):
+    self.obj = OwnerPrintingPIDLockFile('/tmp/test', False)
+
+  def test_cmdline_for_pid(self):
+    self.assertIsInstance(self.obj.cmdline_for_pid(os.getpid()), six.string_types)