Fix version of combine (since beta version updates might be breaking)

Author: tailhook

Cargo.toml

@@ -14,7 +14,7 @@ version = "0.1.0"
 authors = ["[email protected]"]
 
 [dependencies]
-combine = "3.0.0-beta.1"
+combine = "=3.0.0-beta.1"
 failure = "0.1.1"
 
 [dev-dependencies]