hypothesis tests are failing on master

[shoyer]

Example failure: https://travis-ci.org/pydata/xarray/jobs/433231165

============================= test session starts ==============================
platform linux -- Python 3.6.6, pytest-3.8.1, py-1.6.0, pluggy-0.7.1
hypothesis profile 'default' -> database=DirectoryBasedExampleDatabase('/home/travis/build/pydata/xarray/.hypothesis/examples')
rootdir: /home/travis/build/pydata/xarray, inifile: setup.cfg
plugins: cov-2.6.0, hypothesis-3.73.1
collected 0 items / 1 errors                                                   
==================================== ERRORS ====================================
______________ ERROR collecting properties/test_encode_decode.py _______________
properties/test_encode_decode.py:16: in <module>
    settings.deadline = None
../../../miniconda/envs/test_env/lib/python3.6/site-packages/hypothesis/_settings.py:127: in __setattr__
    'to decorate your test instead.' % (name, value)
E   AttributeError: Cannot assign hypothesis.settings.deadline=None - the settings class is immutable.  You can change the global default settings with settings.load_profile, or use @settings(...) to decorate your test instead.
!!!!!!!!!!!!!!!!!!! Interrupted: 1 errors during collection !!!!!!!!!!!!!!!!!!!!
=========================== 1 error in 0.73 seconds ============================

cc @Zac-HD

[Zac-HD]

Wow, this is embarrassing - I strengthened that check because I thought people might be relying on a silent no-op, and the first case I hear about... was me. Um. #2442 fixes it, at least?