TST/DOC: Add a test for vanhove and fix the documented example.

[danielballan]

The problem reported in #394 is still a mystery to me, but it did highlight
that the vanhove function has no test coverage. Here is a simple smoke
test. It passes for me locally, in a fresh environment with the latest version
of all the required dependencies (obtained via pip):

$ pip list
cycler (0.10.0)
kiwisolver (1.0.1)
matplotlib (2.2.3)
nose (1.3.7)
numpy (1.15.0)
pandas (0.23.4)
pip (9.0.1)
pkg-resources (0.0.0)
pyparsing (2.2.0)
python-dateutil (2.7.3)
pytz (2018.5)
PyYAML (3.13)
scipy (1.1.0)
setuptools (39.0.1)
six (1.11.0)
trackpy (0.4.1+26.g964a3ae.dirty, /home/dallan/Repos/bnl/trackpy)
wheel (0.31.1)

The documented example was missing a required argument (lagtime). I fixed
that.

[caspervdw]

@danielballan We need #517 to assess the error in the CI. Btw, good to do some work at trackpy again. We either should try keeping it up or find other contributors.