Fix issue with reading in VO Tables

[astrofrog]

Due to a change in either Numpy or Astropy (haven't had a chance to investigate which), reading of VO Tables with non-float columns was broken. This PR fixes this.

Fixes #1910

[codecov]

Codecov Report

Merging #1911 into master will decrease coverage by 0.11%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #1911      +/-   ##
=========================================
- Coverage   85.62%   85.5%   -0.12%     
=========================================
  Files         236     236              
  Lines       21042   21042              
=========================================
- Hits        18017   17993      -24     
- Misses       3025    3049      +24

Impacted Files	Coverage Δ
glue/core/data_factories/astropy_table.py	91.66% <100%> (+3.33%)	⬆️
glue/conftest.py	58% <0%> (-12%)	⬇️
glue/utils/qt/app.py	86.04% <0%> (-9.31%)	⬇️
glue/core/data_factories/pandas.py	83.01% <0%> (-3.78%)	⬇️
glue/app/qt/preferences.py	91.01% <0%> (-3.38%)	⬇️
glue/_settings_helpers.py	91.17% <0%> (-2.95%)	⬇️
glue/viewers/matplotlib/layer_artist.py	95% <0%> (-2.5%)	⬇️
glue/plugins/exporters/plotly/export_plotly.py	73.71% <0%> (-1.29%)	⬇️
glue/app/qt/application.py	77.77% <0%> (-0.85%)	⬇️

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[astrofrog]

The Travis failure is unrelated