Use URL to access JLDArchives

[timholy]

In preparation for the removal of JLDArchives from the registry. (I don't think it needs to be registered.)

Fixes JuliaIO/JLDArchives.jl#18

[codecov]

Codecov Report

Merging #262 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #262   +/-   ##
=======================================
  Coverage   71.87%   71.87%           
=======================================
  Files           4        4           
  Lines        1522     1522           
=======================================
  Hits         1094     1094           
  Misses        428      428

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