support Rails 6.1.4 by making shared_connection_pool optional

[gilesbowkett]

this commit adds a CYPRESS_RAILS_SHARED_CONNECTION_POOL env var which defaults to true. if you set it to false, you can run cypress-rails with Rails 6.1.4 and no shared connection pool.

caveats:

• https://github.com/testdouble/cypress-rails/pull/89 seems to run fine without this step; I couldn't tell you why.
• I don't have an app handy which uses a shared connection pool to test this PR on.

this is obviously not a perfect solution. my client's app doesn't use a shared connection pool, so it solves the problem for my limited use case, but it may only be useful for this project as a starting point, or as a stopgap, for other users who don't have shared connection pools to consider either.

more discussion of the issue in #88.

[searls]

I believe this should also be obviated by 0.5.1

[gilesbowkett]

👍🏻