Update the benchmark script to run against both backends

Author: sgrif

bin/bench

@@ -1,3 +1,4 @@
 #!/bin/sh
 
-(cd diesel_tests && cargo bench --no-default-features --features unstable)
+(cd diesel_tests && cargo bench --no-default-features --features "unstable sqlite")
+(cd diesel_tests && cargo bench --no-default-features --features "unstable postgres")