$ rvm use 2.5.3 --install --binary --fuzzy
$ export BUNDLE_GEMFILE=$PWD/Gemfile
$ ruby --version
$ gem install bundler -v 1.17.2
$ bundle install --jobs=3 --retry=3
Fetching gem metadata from https://rubygems.org/..........
Fetching gem metadata from https://rubygems.org/.
Resolving dependencies...
Bundler could not find compatible versions for gem "bundler":
In Gemfile:
bundler (~> 1.13)
Current Bundler version:
bundler (2.0.1)
This Gemfile requires a different version of Bundler.
Perhaps you need to update Bundler by running `gem install bundler`?
Could not find gem 'bundler (~> 1.13)' in any of the relevant sources:
the local ruby installation
So removed bundler gem version specifier from gemspec file.
in #15, CI failed.
https://travis-ci.org/groovenauts/capistrano-systemd-multiservice/jobs/660439514?utm_medium=notification&utm_source=github_status
So removed bundler gem version specifier from gemspec file.