Determine database_url when first accessed versus when required

[Blacksmoke16]

Changes how the DATABASE_URL ENV var is loaded so that the value is determined when first used instead of when micrate itself is required. Removes the need to manually overwrite the connection URL if the ENV var is not immediately available. E.g. when set via a dotenv shard.

Before:

ENV["DATABASE_URL"] = "FOO"

require "./src/micrate"

ENV["DATABASE_URL"] = "BAR"

pp Micrate::DB.connection_url # => "FOO"

After:

ENV["DATABASE_URL"] = "FOO"

require "./src/micrate"

ENV["DATABASE_URL"] = "BAR"

pp Micrate::DB.connection_url # => "BAR"

[hugopl]

I'm not a contributor (yet) but these changes looks good to me.