Always check for security token

[jlev]

fix for #282

[codecov-io]

Codecov Report

Merging #508 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #508      +/-   ##
==========================================
+ Coverage   76.44%   76.45%   +0.01%     
==========================================
  Files          11       11              
  Lines        1592     1593       +1     
==========================================
+ Hits         1217     1218       +1     
  Misses        375      375

Impacted Files	Coverage Δ
storages/backends/s3boto3.py	87.16% <100%> (+0.03%)	⬆️

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[jschneier]

Closing in favor of #514.