removing explicit docker pinning

[PatMyron]

introduced in #43, but docker is already an indirect dependency from cloudformation-cli, so this was just pinning docker version stricter than cloudformation-cli, which causes issues
aws-cloudformation/cloudformation-cli#706
aws-cloudformation/cloudformation-cli#727
aws-cloudformation/cloudformation-cli#759
aws-cloudformation/cloudformation-cli-python-plugin#147

[codecov]

Codecov Report

Merging #50 (1446dad) into master (4bcde0a) will not change coverage.
The diff coverage is n/a.

❗ Current head 1446dad differs from pull request most recent head 3b8ea1b. Consider uploading reports for the commit 3b8ea1b to get more accurate results

@@           Coverage Diff           @@
##           master      #50   +/-   ##
=======================================
  Coverage   98.93%   98.93%           
=======================================
  Files          13       13           
  Lines        1215     1215           
  Branches      212      212           
=======================================
  Hits         1202     1202           
  Misses         13       13           

Flag	Coverage Δ
unittests	98.93% <ø> (ø)

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[shadow-light]

Any reason why this hasn't been released yet? Noticed it was committed 5 months ago and just ran into the problem.