Updated README.md

[thermionic]

added enabling Universe for Ubuntu 18.04

Pre-Request Checklist

• Passes rubocop code analysis (try rubocop --auto-correct)
• Tests added or adapted (try rake test)
• Changes are reflected in the documentation
• User-visible changes appended to CHANGELOG.md

Description

[codecov-io]

Codecov Report

Merging #1590 into master will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master   #1590      +/-   ##
=========================================
- Coverage   62.54%   62.5%   -0.04%     
=========================================
  Files          30      30              
  Lines        1495    1491       -4     
=========================================
- Hits          935     932       -3     
+ Misses        560     559       -1

Impacted Files	Coverage Δ
lib/oxidized/output/git.rb	17.32% <0%> (-0.65%)	⬇️
lib/oxidized/input/ssh.rb	54.73% <0%> (-0.48%)	⬇️
lib/oxidized/node.rb	71.32% <0%> (-0.2%)	⬇️
lib/oxidized/hook/githubrepo.rb	75% <0%> (+1.82%)	⬆️

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[laf]

LGTM

[wk]

Thanks!