Point diesel_codegen back at git, use local override for dev

This resolves the issues with compilefail that was caused when two
`diesel` crates got built as transitive dependencies, while still
allowing people to use git dependencies properly.

Cargo documents that "This path must be an absolute path.", but that
appears to be untrue, which is good for our use case.

Author: sgrif

.cargo/config

@@ -0,0 +1 @@
+paths = ["diesel"]

diesel_codegen/Cargo.toml

@@ -20,7 +20,7 @@ quasi = { verision = "^0.3.8", default-features = false }
 quasi_macros = { version = "^0.3.9", optional = true}
 syntex = { version = "^0.22.0", optional = true }
 syntex_syntax = { version = "^0.22.0", optional = true }
-diesel = { path = "../diesel" }
+diesel = { version = "^0.3.0" }
 
 [features]
 default = ["with-syntex"]