Fix deprecation warning from ActiveSupport for Kernel#capture

[vandrijevik]

Fixes the ActiveSupport deprecation warning:

DEPRECATION WARNING: `#capture(stream)` is deprecated and will be removed in the next release. (called from block in <class:MysqlAdapterTest> at /Users/vladimir/code/foreigner/test/foreigner/connection_adapters/mysql_adapter_test.rb:6)

by moving away from capture and using Foreigner::UnitTest#with_stdout. In the process, moves Foreigner::AdapterTest to be a subclass of Foreigner::UnitTest so this method is available.

Note: If this PR is accepted before #172, I will refactor that code to just use the Foreigner::UnitTest#with_stdout method instead of defining it on AdapterTest.

[matthuhiggins]

This has conflicts with the other PR. If you fix these I will merge.

[vandrijevik]

@matthuhiggins Thanks for the quick review and merge of the other two! I fixed the conflict, so this should be good to go.