use formatted number as schema version

[hedgesky]

In short, this PR turns this line in schema.rb:

ActiveRecord::Schema.define(version: 20170404131909)

into this:

ActiveRecord::Schema.define(version: 2017_04_04_131909)

From my point of view, being able to recognize version timestamp at a glance might be very helpful when resolving merge conflicts (which are frequent in schema.rb).

This change is possible because Ruby perfectly understands integers with underscores as delimiters:

2017_04_04_131909 == 20170404131909 # => true

This PR doesn't affect migration naming, only schema dumper.

[rails-bot]

Thanks for the pull request, and welcome! The Rails team is excited to review your changes, and you should hear from @rafaelfranca (or someone else) soon.

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[sandipransing]

I like the idea 👍

[hedgesky]

It seems that build fail isn't related to proposed changes: it's caused by puma. Some kind of ports issues.