update for RubyGems 3.0

.travis.yml

@@ -3,10 +3,15 @@ sudo: false
 group: beta
 language: ruby
 cache: bundler
+
 before_install:
-  # https://github.com/travis-ci/travis-ci/issues/9383#issuecomment-377680108
-  # recent versions of RubyGems include bundler
-  - gem update --system
+  # rubygems 2.7.8 and greater include bundler
+  - |
+    rv="$(ruby -e 'STDOUT.write RUBY_VERSION')";
+    if   [ "$rv" \< "2.3" ]; then gem update --system 2.7.8 --no-document
+    elif [ "$rv" \< "2.6" ]; then gem update --system --no-document --conservative
+    fi
+  - ruby -v && gem --version && bundle version
 
 rvm:
   - 2.2.10