Code cleanup (#2369)

* Lint travis.yml on https://config.travis-ci.com/explore

* Replace deprecated 'thread_safe' with 'concurrent-ruby' alternative

'thread_safe' gem is now deprecated and merged into 'concurrent-ruby'.
Ref: 52e5f373a9 (diff-42d5a45da331eaa07d2b315bd3c9e738)

* Fix deprecation warning for Ruby 2.7

https://bugs.ruby-lang.org/issues/15539


* Remove a TODO tag that is already resolved

.travis.yml

@@ -1,7 +1,7 @@
 language: ruby
-sudo: false
+os: linux
 
-ruby_supported_versions:
+_ruby_supported_versions:
   - &ruby_2_1 2.1.10
   - &ruby_2_2 2.2.10
   - &ruby_2_3 2.3.8
@@ -10,15 +10,15 @@ ruby_supported_versions:
   - &ruby_2_6 2.6.5
   - &ruby_head ruby-head
 
-jruby_supported_versions:
+_jruby_supported_versions:
   - &jruby_9_1 jruby-9.1.17.0
   - &jruby_9_2 jruby-9.2.8.0
   - &jruby_head jruby-head
 
-jdk_supported_versions:
+_jdk_supported_versions:
   - &jdk_8 openjdk8
 
-rails_supported_versions:
+_rails_supported_versions:
   - &rails_4_1 RAILS_VERSION=4.1
   - &rails_4_1_jruby RAILS_VERSION=4.1 JRUBY_OPTS='--dev -J-Xmx1024M --debug'
   - &rails_4_2 RAILS_VERSION=4.2
@@ -41,7 +41,7 @@ cache:
 before_install:
   - "travis_retry gem update --system 2.7.9"
   - "travis_retry gem install bundler -v '1.17.3'"
-install: BUNDLER_VERSION=1.17.3 bundle install --path=vendor/bundle --retry=3 --jobs=3
+install: bundle install --path=vendor/bundle --retry=3 --jobs=3
 
 script:
   - bundle exec rake ci
@@ -49,7 +49,9 @@ after_success:
   - codeclimate-test-reporter
 
 env:
-  matrix:
+  global:
+    - BUNDLER_VERSION=1.17.3
+  jobs:
     - *rails_4_1
     - *rails_4_2
     - *rails_5_0
@@ -70,7 +72,7 @@ rvm:
 branches:
   only: 0-10-stable
 
-matrix:
+jobs:
   include:
     - { rvm: *jruby_9_1,  jdk: *jdk_8, env: *rails_4_1_jruby }
     - { rvm: *jruby_9_1,  jdk: *jdk_8, env: *rails_4_2_jruby }