changing to new openmm version

[JohannesKarwou]

In the test_mutation file, I get an error for all test using parmed:

    @wraps(fcn)
    def new_fcn(*args, **kwargs):
        if not HAS_OPENMM:
>           raise ImportError('Could not find or import OpenMM version 7.0+')
E           ImportError: Could not find or import OpenMM version 7.0+

In the parmed/utils/decorators.py, parmed checks for the openmm version with:

try:
    import openmm as mm

    HAS_OPENMM = True
except ImportError:
    HAS_OPENMM = False

which will not find openmm v.7.5.1 where one needs to use import simtk.openmm