Remove Ruby 2.3, 2.4 and 2.5 from the CI test matrix

[daniel-niknam-envato]

Context

Ruby 2.3, 2.4 and 2.5 have reached their EOL. We removing dropping test matrixes for those ruby versions.

Considaration

• We were unable to find an Envato application using this gem with one of the above ruby versions
• We will reach out to Envato engineering before releasing a new gem
• A new version will be released in the next PR

[daniel-niknam-envato]

Postgres gem is also updated: https://github.com/envato/event_sourcery-postgres/pull/70