Use SPDX license format

The use of `/` have been deprecated

Author: jbtrystram

Cargo.toml

@@ -2,7 +2,7 @@
 name = "native-tls"
 version = "0.2.11"
 authors = ["Steven Fackler <[email protected]>"]
-license = "MIT/Apache-2.0"
+license = "MIT OR Apache-2.0"
 description = "A wrapper over a platform's native TLS implementation"
 repository = "https://github.com/sfackler/rust-native-tls"
 readme = "README.md"