use ActiveRecord::Migration version 5.0 or 5.1 instead of 4.2

[wayne5540]

Why

1. Since this gem is dropping support for Rails 4.2 (Rails 5.2 support, drop Rails 4.2 support. #887), there is no reason to use ActiveRecord::Migration[4.2] as default migration version, sooner we migrate to 5 better
2. In Rails version > 5.1, default type of primary key becomes bigint, use ActiveRecord::Migration[5.1] can also help us adapt this change, make it more consistent

Notable point

1. In Rails version > 5, migration method t.references will add index automatically, to keep migration result consistent, I add index: false for tag, taggable , tagger to prevent extra index_taggings_on_taggable_type_and_taggable_id and index_taggings_on_tagger_type_and_tagger_id index
2. After this change, if your ActiveRecord version > 5.1, all table's id will become bigint (if your database supports bigint)

[nbulaj]

Why not just use [#{ActiveRecord::VERSION::MAJOR}.#{ActiveRecord::VERSION::MINOR}] to support any AR version?

[wayne5540]

@nbulaj Yes, agree with you, since .gemspec now has run time activerecord
dependency >= 5.0, < 6.1, we probably don't need the if else statement anymore and we can just use [#{ActiveRecord::VERSION::MAJOR}.#{ActiveRecord::VERSION::MINOR}], thanks for pointing this out.

I'm happy to do the change, however, this PR has been idle here for a while, I'm not sure if this change is wanted 😅

[nbulaj]

I'm happy to do the change, however, this PR has been idle here for a while, I'm not sure if this change is wanted sweat_smile

You need to ask them, try to ping @seuros :)

[seuros]

@wayne5540 Thank you for your contribution, can you rebase this PR and have it use version 6.0 instead.

[wayne5540]

Looks like all the files has been updated to 6.0 version at master branch, nice work team! This PR is no longer needed.