Restrict doctests to Julia 1.5

[mjram0s]

Resolves #102

[codecov]

Codecov Report

Merging #103 (019a35c) into master (5defe62) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #103   +/-   ##
=======================================
  Coverage   97.56%   97.56%           
=======================================
  Files           6        6           
  Lines         205      205           
=======================================
  Hits          200      200           
  Misses          5        5           

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[rofinn]

Looks like the Pkg.activate example is having trouble installing TimeZones on 1.6. Would you mind keeping this at 1.5 and restrict the documentation job as well?