Use path overrides for the full workspace

By adding the `[replace]` here, the override acts more like the path
overrides in `.cargo/config` used to. This ensures that all member
crates are using the local version when running tests. I'm not sure
if/how this is used when publishing to crates.io, but presumably it'll
error if it's not ignored. I guess I'll find out when we release later
this week.

Author: sgrif

Cargo.toml

@@ -13,3 +13,9 @@ members = [
     "examples/getting_started_step_3",
     "examples/getting_started_step_4",
 ]
+
+[replace]
+"diesel:0.8.2" = { path = "diesel" }
+"diesel_codegen:0.8.2" = { path = "diesel_codegen" }
+"diesel_codegen_shared:0.8.0" = { path = "diesel_codegen_shared" }
+"diesel_codegen_syntex:0.8.0" = { path = "diesel_codegen_syntex" }