Update minitest to avoid warnings

"MiniTest::Unit::TestCase is now Minitest::Test."

Change-Id: Ib78b7b0b41581e1195083c1e8ca54bda46352eca
Reviewed-on: http://review.couchbase.org/26885
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>