Fix this problem reported by Travis-CI:

ERROR: Test that altering the mass of hydrogens works correctly.
----------------------------------------------------------------------
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

wrappers/python/simtk/openmm/app/charmmpsffile.py

@@ -774,7 +774,7 @@ class CharmmPsfFile(object):
                 elem = element.Element.getByAtomicNumber(atomic_num)
             else:
                 # Figure it out from the mass
-                masselem = Element.getByMass(mass)
+                masselem = element.Element.getByMass(mass)
                 if masselem is None:
                     atomic_number = 0 # Extra point or something
                 else: