Use puts instead of p for multi-line message

[vandrijevik]

Kernel#p calls #inspect on its argument before writing it to the output, so the warning message for an unsupported adapter is displayed like this:

"Database adapter seamless_database_pool not supported. Use:\nForeigner::Adapter.register 'seamless_database_pool', 'path/to/adapter'"

This PR changes the p to a puts, so the message displayed is:

Database adapter seamless_database_pool not supported. Use:
Foreigner::Adapter.register 'seamless_database_pool', 'path/to/adapter'

as intended.

Note: capture(stream) is deprecated, which is why the test uses a StringIO to capture the output instead of calling that method.