Handle deprecation of assert_block safely

MiniTest 4.6.0 and higher no longer has assert_block as a method [opting for assert(yield, message)] https://github.com/seattlerb/minitest/blob/master/History.txt#L134-L140

This enables users with older MiniTest to still use assert_block if it's there and the newer format if it's not.

Not sure how to write a test for this as it is a dependency-based issue.