Use Conda for ReadTheDocs build to install numpy/scipy

[winstonaws]

Previously, our RTD builds were failing due to scipy/numpy dependencies declared in install_requires: https://readthedocs.org/projects/sagemaker/builds/7040261/

With this change, we now use conda for the RTD build environment, which allows us to install the dependencies.

Successful RTD build after changes:
https://readthedocs.org/projects/sagemaker/builds/7050841/

Docs after the changes:
http://sagemaker.readthedocs.io/en/rtd_debug/estimators.html

Based the changes off this issue: readthedocs/readthedocs.org#2549

And this repo, which the issue cites as an example: https://github.com/astropy/astropy

Merge Checklist

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• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works (if appropriate)
• I have updated the changelog with a description of my changes (if appropriate)
• I have updated any necessary documentation (if appropriate)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-io]

Codecov Report

Merging #150 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #150   +/-   ##
=======================================
  Coverage   90.48%   90.48%           
=======================================
  Files          36       36           
  Lines        2438     2438           
=======================================
  Hits         2206     2206           
  Misses        232      232

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[ChoiByungWook]

Good job!