Fix travis test failures

[davidlmobley]

Travis tests are failing with issues like:

======================================================================
ERROR: Test PhysicalPropertyEstimator on synthetic data.
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/travis/miniconda/lib/python2.7/site-packages/nose/case.py", line 197, in runTest
    self.test(*self.arg)
  File "/home/travis/miniconda/lib/python2.7/site-packages/openforcefield/tests/test_propertyestimator.py", line 13, in test_propertyestimator
    from openforcefield import testsystems
ImportError: cannot import name testsystems
======================================================================
ERROR: Failure: ImportError (cannot import name PhysicalPropertyDataset)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/travis/miniconda/lib/python2.7/site-packages/nose/loader.py", line 418, in loadTestsFromName
    addr.filename, addr.module)
  File "/home/travis/miniconda/lib/python2.7/site-packages/nose/importer.py", line 47, in importFromPath
    return self.importFromDir(dir_path, fqname)
  File "/home/travis/miniconda/lib/python2.7/site-packages/nose/importer.py", line 94, in importFromDir
    mod = load_module(part_fqname, fh, filename, desc)
  File "/home/travis/miniconda/lib/python2.7/site-packages/openforcefield/tests/testsystems.py", line 35, in <module>
    from openforcefield import PhysicalPropertyDataset
ImportError: cannot import name PhysicalPropertyDataset
openforcefield.tests.test_propertyestimator.test_propertyestimator: 0.0002s
nose.failure.Failure.runTest: 0.0001s
----------------------------------------------------------------------
Ran 2 tests in 0.001s
FAILED (errors=2)
The command "cd devtools && nosetests -vv --nocapture --with-timer $PACKAGENAME && cd .." exited with 1.
Done. Your build exited with 1.

I need to deal with these. We're not actively doing that much with this repo but Bryce is working on stuff so it would be good to have the tests working for him.