Update .travis.yml (JabRef#4970)

Author: rmzelle

Diff for: .travis.yml

@@ -1,8 +1,8 @@
-dist: bionic
+dist: focal
 language: ruby
 cache: bundler
 rvm:
-- 2.6.5
+- 2.7.1
 install:
 - bundle install --jobs=3 --retry=3 --path=${BUNDLE_PATH:-vendor/bundle}
 - bundle update sheldon

Diff for: Gemfile

@@ -1,4 +1,4 @@
-ruby '2.6.5'
+ruby '2.7.1'
 source 'https://rubygems.org'
 
 gem 'rake'

Diff for: Gemfile.lock

@@ -64,7 +64,7 @@ DEPENDENCIES
   sheldon!
 
 RUBY VERSION
-   ruby 2.6.5p114
+   ruby 2.7.1
 
 BUNDLED WITH
-   1.17.3
+   2.1.4