Switch to using xenial build environment on Travis.

.travis.yml

@@ -1,4 +1,4 @@
-sudo: false
+dist: xenial
 language: python
 python:
   - "2.7"
@@ -12,7 +12,7 @@ install:
   - source venv/bin/activate
   - |
     if [[ $TRAVIS_PYTHON_VERSION == 3.7 ]]; then
-        pip install "cothread==2.16"
+        pip install git+https://github.com/dls-controls/cothread.git
     fi
   - pip install -r requirements.txt
   - pip install git+https://github.com/dls-controls/pytac.git

atip/at_interface.py

@@ -60,7 +60,7 @@ def __init__(self, at_lattice, callback=None):
             raise TypeError("If passed, 'callback' should be callable, {0} is "
                             "not.".format(callback))
         self._at_lat = at_lattice
-        self._rp = numpy.ones(len(at_lattice), dtype=bool)  # faster option?
+        self._rp = numpy.ones(len(at_lattice), dtype=bool)
         # Initial phys data calculation.
         self._at_lat.radiation_on()
         self._emittance = self._at_lat.ohmi_envelope(self._rp)