Squash the following warning:

/usr/local/openmm/include/openmm/CustomHbondForce.h:477:25: warning: field 'p3' will be initialized after field 'parameters' [-Wreorder]
        p1(p1), p2(p2), p3(p3), parameters(parameters) {
                            ^
I believe that the initializer is supposed to assign variables in the same
order they're declared in the class definition (not the same order they're
defined in the constructor argument list.
Merge pull request #760 from swails/master

Squash the following warning: