Lock Bundler to 1.x

Bundler 2.0 was released recently, but none of our lockfiles are
associated with this version — they all refer to Bundler 1.x.
Unfortunately, Travis is now attempting to use Bundler 2.0 by default,
so we have to ensure that it is using the correct version as well.

.travis.yml

@@ -2,6 +2,10 @@ language: ruby
 sudo: false
 cache: bundler
 script: "bundle exec rake"
+# Source: <https://docs.travis-ci.com/user/languages/ruby/#bundler-20>
+before_install:
+  - gem uninstall -v '>= 2' -i $(rvm gemdir)@global -ax bundler || true
+  - gem install bundler -v '< 2'
 install: "bundle install --jobs=3 --retry=3"
 
 env:

bin/setup

@@ -164,7 +164,7 @@ install-dependencies() {
   fi
 
   banner 'Installing Ruby dependencies'
-  gem install bundler --conservative
+  gem install bundler -v '~> 1.0' --conservative
   bundle check || bundle install
   bundle exec appraisal install