integration tests in CI use a travis provided python that doesn't contain setuptools

[kwlzn]

discovered while working through #461:

=================================== FAILURES ===================================
_________________ test_interpreter_constraints_to_pex_info_py3 _________________
    @pytest.mark.skipif(NOT_CPYTHON_36)
    def test_interpreter_constraints_to_pex_info_py3():
      with temporary_dir() as output_dir:
        # target python 3
        pex_out_path = os.path.join(output_dir, 'pex_py3.pex')
        res = run_pex_command(['--disable-cache',
          '--interpreter-constraint=>3',
          '-o', pex_out_path])
>       res.assert_success()
tests/test_integration.py:354: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
self = IntegResults(output=['Saving PEX file to /tmp/tmphjizs6w3/pex_py3.pex'], return_code=None, exception=RuntimeError('Fai...\' % self._interpreter.binary\nRuntimeError: Failed to find setuptools via /usr/bin/python3.2mu while building pex!\n')
    def assert_success(self):
      if not (self.exception is None and self.return_code is None):
        raise AssertionError('integration test failed: return_code=%s, exception=%r, traceback=%s' % (
>         self.return_code, self.exception, self.traceback
        ))
E       AssertionError: integration test failed: return_code=None, exception=RuntimeError('Failed to find setuptools via /usr/bin/python3.2mu while building pex!',), traceback=Traceback (most recent call last):
E         File "/home/travis/build/kwlzn/pex/.tox/py36-requests/lib/python3.6/site-packages/pex/testing.py", line 241, in run_pex_command
E           main(args=args)
E         File "/home/travis/build/kwlzn/pex/.tox/py36-requests/lib/python3.6/site-packages/pex/bin/pex.py", line 669, in main
E           pex_builder.build(tmp_name)
E         File "/home/travis/build/kwlzn/pex/.tox/py36-requests/lib/python3.6/site-packages/pex/pex_builder.py", line 472, in build
E           self.freeze(bytecode_compile=bytecode_compile)
E         File "/home/travis/build/kwlzn/pex/.tox/py36-requests/lib/python3.6/site-packages/pex/pex_builder.py", line 456, in freeze
E           self._prepare_bootstrap()
E         File "/home/travis/build/kwlzn/pex/.tox/py36-requests/lib/python3.6/site-packages/pex/pex_builder.py", line 409, in _prepare_bootstrap
E           'Failed to find setuptools via %s while building pex!' % self._interpreter.binary
E       RuntimeError: Failed to find setuptools via /usr/bin/python3.2mu while building pex!
.tox/py36-requests/lib/python3.6/site-packages/pex/testing.py:214: AssertionError

I'm going to skip the two affected tests for now.