Fix use of deprecated numpy dtypes

[landmanbester]

• [ ] Tests added / passed

  $ py.test --flake8 -v -s .

  If the flake8 tests fail, the quickest way to correct this is to run autopep8 and then flake8 to fix the remaining issues.

  $ pip install -U autopep8 flake8
  $ autopep8 -r -i .
  $ flake8 .

Fixes https://github.com/caracal-pipeline/crystalball/issues/61

[sjperkins]

Looks good, is there anything else needed here before merge?

[landmanbester]

I don't think so. It works for me at least