Fix tests

[davimacedo]

This PR adds a fake aws sdk for running unit tests when the aws credentials are not available (for instance, in travis).

It is a very simple fake adapter, but if needed, later, we can keep improving it to simulate real issues.

Tests with the real aws sdk can still be done locally by setting up the TEST_S3_ACCESS_KEY, TEST_S3_SECRET_KEY, and TEST_S3_BUCKET env vars.

Close #67

[codecov]

Codecov Report

Merging #68 into master will increase coverage by 22.91%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master      #68       +/-   ##
===========================================
+ Coverage   70.83%   93.75%   +22.91%     
===========================================
  Files           2        2               
  Lines         144      144               
  Branches       31       31               
===========================================
+ Hits          102      135       +33     
+ Misses         42        9       -33

Impacted Files	Coverage Δ
index.js	92.2% <0%> (+42.85%)	⬆️

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