Fix pairing benchmarks

They were broken by #91 but went unnoticed because CI does not compile
the benchmarks, which requires the nightly toolchain.

pairing/benches/pairing_benches.rs

@@ -3,7 +3,8 @@
 extern crate ff;
 extern crate group;
 extern crate pairing;
-extern crate rand;
+extern crate rand_core;
+extern crate rand_xorshift;
 extern crate test;
 
 mod bls12_381;