python 2.7 numpy 1.16 errors

[wholmgren]

https://travis-ci.org/pvlib/pvlib-python/jobs/480589919

Could be that we're using numpy functions in an unsupported way and they only work on some platforms/versions. Could also be related to the conda-forge builds.

[wholmgren]

A few things going on here:

1. On python 2.7 numpy 1.16, for scalar input, np.digitize now returns a python int instead of an array with ndim == 0. This breaks the perez function with scalar input.
2. A test (numba + calc delta t) that was expected to fail due to a ValueError now fails for a different reason. We never promised any specific failure here and I don't think anyone relies on the behavior. So, I suggest relaxing the xfail condition to any error.
3. numpy 1.16 deprecates np.asscalar, so the logs are littered with deprecationwarnings.

PR incoming.