[#7] Convert to Float before rounding; avoid a segfault

For a covered percent of Rational(200/10)
covered_percent = Marshal.load("\x04\bU:\rRational[\ai\x01\xC8i\x0F")
covered_percent.round(2) => segfault
Float(covered_percent).round(2) => 20.0

The segfault depends on as yet unknown gems to be required