Bump to ruby 2.6.1 and update all deps

[meatballhat]

including re-rubocop, removing jemalloc, and adding codecov (finally?)

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@416fb36). Click here to learn what that means.
The diff coverage is 0%.

@@            Coverage Diff            @@
##             master      #84   +/-   ##
=========================================
  Coverage          ?   92.64%           
=========================================
  Files             ?       31           
  Lines             ?     1034           
  Branches          ?        0           
=========================================
  Hits              ?      958           
  Misses            ?       76           
  Partials          ?        0

Impacted Files	Coverage Δ
lib/job_board/config.rb	82.75% <0%> (ø)

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