Auto merge of #299 - dotdash:rational-fix, r=cuviper
Auto merge of #299 - dotdash:rational-fix, r=cuviper

Fix building without the num-bigint feature

rust-num/num,rust-num/num-bigint,rust-num/num-integer,rust-num/num,rust-lang/num,rust-lang/num,koverstreet/num,koverstreet/num,rust-num/num-integer,rust-num/num-traits,rust-num/num-bigint,rust-num/num-traits