devel/rubygem-strong_migrations61: New port

This gem catches the following unsafe migrations:
  - adding a column with a non-null default value to an existing table
  - changing the type of a column
  - renaming a table
  - renaming a column
  - removing a column
  - adding an index non-concurrently (Postgres only)
  - adding a json column to an existing table (Postgres only)

Sponsored by:	Nepustil