add chainer, bump to 1.3

[andremoeller]

Issue #, if available:

Description of changes: Adding chainer estimator, bumping version to 1.3

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 #195 into master will increase coverage by 0.34%.
The diff coverage is 99.09%.

@@            Coverage Diff             @@
##           master     #195      +/-   ##
==========================================
+ Coverage   90.68%   91.03%   +0.34%     
==========================================
  Files          38       42       +4     
  Lines        2599     2699     +100     
==========================================
+ Hits         2357     2457     +100     
  Misses        242      242

Impacted Files	Coverage Δ
src/sagemaker/predictor.py	96.83% <100%> (+1.24%)	⬆️
src/sagemaker/chainer/estimator.py	100% <100%> (ø)
src/sagemaker/chainer/__init__.py	100% <100%> (ø)
src/sagemaker/content_types.py	100% <100%> (ø)	⬆️
src/sagemaker/chainer/defaults.py	100% <100%> (ø)
src/sagemaker/fw_utils.py	98.52% <100%> (ø)	⬆️
src/sagemaker/chainer/model.py	96.55% <96.55%> (ø)
... and 1 more

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