Fix this problem reported by Travis-CI:

ERROR: Test that altering the mass of hydrogens works correctly.
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Traceback (most recent call last):
  File "/home/travis/build/SimTk/openmm/python/tests/TestCharmmFiles.py", line 91, in test_HydrogenMass
    topology = self.psf_v.topology
  File "/usr/local/lib/python2.7/dist-packages/simtk/openmm/app/charmmpsffile.py", line 777, in topology
    masselem = Element.getByMass(mass)
NameError: global name 'Element' is not defined