Use 'application/sparql-results+json' as default for query

Addressed the issue dbcls/sparql-proxy#12 (comment)

Catch up with Ruby and ActiveSupport updates

• Support ruby 2.5, 2.6 and 2.7
• Drop support ruby older than or equal to 2.4
• Support activesupport 5.x and 6.x
• Drop support activesupport older than or equal to 4.x