Rails 6 support (patch only)

This is a patch bump before we properly update statesman to drop support for older Rails/Ruby versions in the upcoming 5.0.0 release.

• Add support for Rails 6 @greysteil

Bug fixes for index detection and metadata access

• Fix statesman index detection for indexes that start t-z @hmarr
• Correct access of metadata via state_machine @glenpike

Add better support for mysql (and others) in transition_conflict_error?

• #342

Use real transactions inside statesman

• #338 - Fix issue with after_commit transitions being run even if the transaction fails.
• #249 - Enforces nested transactions inside statesman

Expose most_recent_transition_join

See #330

v3.4.1

• When unsetting the most_recent flag during a transition, don't assume that transitions have an updated_at attribute, but rather allow the "updated timestamp column" to be re-configured or disabled entirely (patch by @timrogers)

v3.4.0

• When unsetting the most_recent flag during a transition, don't assume that transitions have an updated_at attribute, but rather allow the "updated timestamp column" to be re-configured or disabled entirely (patch by @timrogers)

v3.3.0

• Touch updated_at on transitions when unsetting most_recent flag (patch by @NGMarmaduke)
• Fix force_reload for ActiveRecord models with loaded transitions (patch by @jacobpgn)

v3.2.0

• Allow specifying metadata with Machine#allowed_transitions
  (patch by @vvondra)

v3.1.0

• Add support for Rails 5.0.x and 5.1.x (patch by @kenchan0130 and @timrogers)
• Run tests in CircleCI instead of TravisCI (patch by @timrogers)
• Update Rubocop and fix offences (patch by @timrogers)