Add lxml to travis

Author: jchodera

.travis.yml

@@ -82,7 +82,7 @@ install:
 - python setup.py develop --no-deps
 
 # Install openmm-forcefields via pip (until available via omnia)
-- conda install --yes tinydb "openforcefield>=0.6.0" "openforcefields>=1.0.0"
+- conda install --yes tinydb lxml "openforcefield>=0.6.0" "openforcefields>=1.0.0"
 - pip install git+https://github.com/openmm/openmm-forcefields.git
 # Workaround for https://github.com/openmm/openmm/pull/2511
 # TODO: Remove this when OpenMM builds including https://github.com/openmm/openmm/pull/2511 are released