There seems to be a problem importing the global gemset on Ruby 2.6.3 on Trusty. See this build for more details. I think the relevant log output is as follows:
$ rvm use 2.6.3 --install --binary --fuzzy
curl: (22) The requested URL returned error: 404 Not Found
Required ruby-2.6.3 is not installed - installing.
curl: (22) The requested URL returned error: 404 Not Found
Searching for binary rubies, this might take some time.
Found remote file https://rubies.travis-ci.org/ubuntu/14.04/x86_64/ruby-2.6.3.tar.bz2
Checking requirements for ubuntu.
Requirements installation successful.
ruby-2.6.3 - #configure
ruby-2.6.3 - #download
% Total % Received % Xferd Average Speed Time Time Time Current
Dload Upload Total Spent Left Speed
0 0 0 0 0 0 0 0 --:--:-- --:--:-- --:--:-- 0
100 15.3M 100 15.3M 0 0 975k 0 0:00:16 0:00:16 --:--:-- 1538k
No checksum for downloaded archive, recording checksum in user configuration.
ruby-2.6.3 - #validate archive
ruby-2.6.3 - #extract
ruby-2.6.3 - #validate binary
ruby-2.6.3 - #setup
ruby-2.6.3 - #gemset created /home/travis/.rvm/gems/ruby-2.6.3@global
ruby-2.6.3 - #importing gemset /home/travis/.rvm/gemsets/global.gems
there was an error installing gem gem-wrappers
there was an error installing gem rubygems-bundler
there was an error installing gem rvm
.........................................................
ruby-2.6.3 - #generating global wrappers..............
Error running 'run_gem_wrappers_regenerate',
please read /home/travis/.rvm/log/1560965486_ruby-2.6.3/gemset.wrappers.global.log
ruby-2.6.3 - #gemset created /home/travis/.rvm/gems/ruby-2.6.3
ruby-2.6.3 - #importing gemset /home/travis/.rvm/gemsets/default.gemsthere was an error installing gem bundler
......................
ruby-2.6.3 - #generating default wrappers..............
Error running 'run_gem_wrappers_regenerate',
please read /home/travis/.rvm/log/1560965488_ruby-2.6.3/gemset.wrappers.default.log
Using /home/travis/.rvm/gems/ruby-2.6.3
$ export BUNDLE_GEMFILE=$PWD/Gemfile
ruby.versions
$ ruby --version
ruby 2.6.3p62 (2019-04-16 revision 67580) [x86_64-linux]
$ rvm --version
rvm 1.29.3 (latest) by Michal Papis, Piotr Kuczynski, Wayne E. Seguin [https://rvm.io]
$ bundle --version
/home/travis/.rvm/rubies/ruby-2.6.3/lib/ruby/2.6.0/rubygems.rb:283:in `find_spec_for_exe': Could not find 'bundler' (2.0.2) required by your /home/travis/build/floehopper/jamesmead.org/Gemfile.lock. (Gem::GemNotFoundException)
To update to the latest version installed on your system, run `bundle update --bundler`.
To install the missing version, run `gem install bundler:2.0.2`
from /home/travis/.rvm/rubies/ruby-2.6.3/lib/ruby/2.6.0/rubygems.rb:302:in `activate_bin_path'
from /home/travis/.rvm/rubies/ruby-2.6.3/bin/bundle:30:in `<main>'
$ gem --version
3.0.3
3.25s$ bundle install --jobs=3 --retry=3 --deployment
/home/travis/.rvm/rubies/ruby-2.6.3/lib/ruby/2.6.0/rubygems.rb:283:in `find_spec_for_exe': Could not find 'bundler' (2.0.2) required by your /home/travis/build/floehopper/jamesmead.org/Gemfile.lock. (Gem::GemNotFoundException)
To update to the latest version installed on your system, run `bundle update --bundler`.
To install the missing version, run `gem install bundler:2.0.2`
from /home/travis/.rvm/rubies/ruby-2.6.3/lib/ruby/2.6.0/rubygems.rb:302:in `activate_bin_path'
from /home/travis/.rvm/rubies/ruby-2.6.3/bin/bundle:30:in `<main>'
The command "eval bundle install --jobs=3 --retry=3 --deployment " failed. Retrying, 2 of 3.