Merge pull request 'Remove Travis CI config, set up Drone CI' (#2) from dev/drone_ci into chore/upgrade_dependencies

Reviewed-on: #2

.drone.yml

@@ -0,0 +1,9 @@
+kind: pipeline
+name: default
+
+steps:
+- name: specs
+  image: ruby
+  commands:
+  - bundle install --jobs=3 --retry=3
+  - rake spec

.travis.yml

@@ -1,6 +0,0 @@
-sudo: false
-language: ruby
-rvm:
-  - 2.6.6
-cache: bundler
-before_install: gem install bundler