pessimistic net-telnet lock on 0.1 as 0.2 requires ruby 2.3+

[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

Pessimistic lock for net-telnet to 0.1 as 0.2 depends on Ruby 2.3+
Make dependency on net-telnet unconditional since the gemspec is evaluated as bundle time - avoids release master environment drift from having impact on published gems (0.21.0 release had no net-telnet dependency in the published gem, but 0.22.0+ does).

Closes #1472

[codecov-io]

Codecov Report

Merging #1475 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1475   +/-   ##
=======================================
  Coverage   62.72%   62.72%           
=======================================
  Files          30       30           
  Lines        1481     1481           
=======================================
  Hits          929      929           
  Misses        552      552

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