Fix worker memory limit to entityset size comparison

[rwedge]

There was a bug in #208 causing featuretools to treat the current memory use of a dask worker as the workers' total usable memory when deciding if the worker had enough memory to store an EntitySet.

[codecov-io]

Codecov Report

Merging #214 into master will decrease coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #214      +/-   ##
==========================================
- Coverage   93.48%   93.46%   -0.03%     
==========================================
  Files          71       71              
  Lines        7757     7784      +27     
==========================================
+ Hits         7252     7275      +23     
- Misses        505      509       +4

Impacted Files	Coverage Δ
featuretools/computational_backends/utils.py	94.9% <100%> (ø)	⬆️
featuretools/tests/testing_utils/cluster.py	100% <100%> (ø)	⬆️
...turetools/computational_backends/pandas_backend.py	91.29% <0%> (-0.44%)	⬇️
...tests/computational_backend/test_pandas_backend.py	100% <0%> (ø)	⬆️

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