Set KERAS_BACKEND=theano before importing DDE

[mliu49]

Motivation or Problem

This ensures that the keras backend is set to theano so that dde works properly, regardless of the environment variables set originally. Fixes #1532.

Description of Changes

Change the KERAS_BACKEND environment variable using os.environ.

Testing

Should be tested on mac:

• Activate rmg_env
• Check echo $KERAS_BACKEND, should be tensorflow
• Verify that make test passes

[codecov]

Codecov Report

Merging #1535 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1535      +/-   ##
==========================================
+ Coverage   42.06%   42.07%   +<.01%     
==========================================
  Files         165      165              
  Lines       27823    27824       +1     
  Branches     5668     5668              
==========================================
+ Hits        11705    11706       +1     
  Misses      15332    15332              
  Partials      786      786

Impacted Files	Coverage Δ
rmgpy/ml/estimator.py	97.56% <100%> (+0.06%)	⬆️

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