Merge pull request #124 from jchodera/master

Switched .travis.yml to use conda package for openmm
Fixed bugs in newly added CustomLennardJonesFluidMixed, which attempts to mix NonbondedForce and CustomNonbondedForce.
Modified CustomLennardJonesFluidMixture so that half of particles are CustomNonbondedForce and other half are NonbondedForce.
Added test to make sure system could be serialized and deserialized to produce the same energies and forces.
Add a Barrier to dummympi
add barrier, too
Enhance dummympi a bit.
Enhance the dummy MPI implementation a little bit.
Add passes.
Fix up doctest example.
Merge pull request #127 from swails/master

Beef up the dummy MPI machinery
Fix travis-ci by updating openmm conda package location.
Merge remote-tracking branch 'upstream/master'