Drop support for Ruby 2.3 which reached EOL last year

.travis.yml

@@ -30,7 +30,6 @@ matrix:
     - rvm: 2.6.5
       env: DATABASE_ADAPTER=mysql2 DOCKER_IMAGE=huginn/huginn DOCKERFILE=docker/multi-process/Dockerfile
 rvm:
-- 2.3.8
 - 2.4.9
 - 2.5.7
 - 2.6.5

Gemfile

@@ -1,6 +1,6 @@
 source 'https://rubygems.org'
 
-ruby '>=2.3.0'
+ruby '>=2.4.0'
 
 # Ensure github repositories are fetched using HTTPS
 git_source(:github) do |repo_name|