Add lockfiles to gitignore

[mlarraz]

These don't really make sense for a library, as they are testing just one out of many possible manifest configurations.

This has the inadvertent effect of fixing the build, as it would currently be broken due to
mimemagicrb/mimemagic#97 which led to a dependency being yanked from RubyGems

[ghost]

All CLA requirements met.

[serprex]

I'm not sure, there's arguments to keep a lockfile, you point out in this case CI being fixed by bumping versions, but it's also ideal that a new gem version doesn't break CI

[serprex]

Thinking over, I'm okay merging this if CI is passing. If some new version causes regression in CI that's an issue where gemfile should have version constraint, not lockfile

[mlarraz]

After investigating, the tests are failing on Rails 5.2 because of some change introduced in ActiveRecord between 5.2.3 and 5.2.4.

I haven't been able to pinpoint exactly what, but it seems to result in subtle changes in query structure:

-- 5.2.3, what the tests expect
SELECT "projects".* FROM "projects"
INNER JOIN "project_categories"
ON "project_categories"."project_id" = "projects"."id"
AND "project_categories"."account_id" = "projects"."account_id" -- no longer present in 5.2.4
INNER JOIN "categories"
ON "categories"."id" = "project_categories"."category_id"
WHERE "projects"."account_id" = 1

-- 5.2.4
SELECT "projects".* FROM "projects"
INNER JOIN "project_categories"
ON "project_categories"."project_id" = "projects"."id"
INNER JOIN "categories"
ON "categories"."id" = "project_categories"."category_id"
WHERE "projects"."account_id" = 1

I don't know enough this library to be able to do much more right now

[serprex]

Tried looking over commits between release of 5.2.3 & 5.2.4, maybe related to rails/rails#37359