cargo: use OR in the license field

PR #131 
diff --git a/Cargo.toml b/Cargo.toml
index 1ada583..b9ff700 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -9,7 +9,7 @@
 readme = "README.md"
 categories = ["encoding", "parsing"]
 keywords = ["byte", "endian", "big-endian", "little-endian", "binary"]
-license = "Unlicense/MIT"
+license = "Unlicense OR MIT"
 
 [lib]
 name = "byteorder"