Check the asv benchmarks

[aragilar]

This adds asv check to (quoting command) "Import and check benchmark suite, but do not run benchmarks." I think this should mean we're checking for syntax errors and such, but not actually running the benchmarks (which might be a bit slow...). I've put this on travis just in case it does take some time, but there's nothing special about the python or HDF5 version used.

[takluyver]

Failing because it can't find Cython when trying to build. I think Cython gets installed in the tox environment where asv is run, but not in the environment asv creates to install h5py. However, it looks like asv disables pip's build isolation, so it's not getting another level of environment isolation there.

Possibly relevant: airspeed-velocity/asv#903