Rational that reduce to ints should print like ints

When you print rational expressions it currently looks bad to see integers formatted as fractions with a denominator of 1 (even though that's how they are stored internally).
For instance, in http://is.gd/0gRT1T, 

    println!("{}", Ratio::<int>::from_integer(42)); // currently prints "42/1"
                                                                         // this PR allows it to print as "42"


tried it and you're right. Plus it looks much more readable too... Thanks!