update installation instructions for latest version

[wk]

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

Validated installation instructions for version 0.23.0 on the following docker containers:

• debian:stretch
• centos:6
• centos:7

Updated installation instructions where applicable.

Closes ytti/oxidized-web#171

[codecov-io]

Codecov Report

Merging #1383 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1383   +/-   ##
=======================================
  Coverage   62.49%   62.49%           
=======================================
  Files          28       28           
  Lines        1397     1397           
=======================================
  Hits          873      873           
  Misses        524      524

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 145b2bd...d16d334. Read the comment docs.