changed type specifier for coercing ratio to float

As the ".. printer uses *read-default-float-format* to guide the choice
of exponent markers when printing floating-point numbers .."
(see: http://www.lispworks.com/documentation/HyperSpec/Body/v_rd_def.htm#STread-default-float-formatST),
using *read-default-float-format* is a sensible default when converting ratios to floats.