Cargo.toml: Replace '/' with 'OR' in 'license'

Catch up with the recommendations from rust-lang/cargo@7dee65fe
(src/doc/manifest: Pin 'license' to SPDX 2.1 expressions and the 2.4
list, 2018-01-04, rust-lang/cargo#4898), which deprecated '/' in favor
of vanilla SPDX license expressions.

I've gone with the disjunctive OR, and most contributors have signed
off on that change:

* Without Boats wanted to match the crates.io licensing [1], and they
  meant OR [2].
* Rodolphe Breard [3] and Laurence Tratt [4] intended OR.

We haven't heard back from Kai NODA, but ed9fe0b (README.md: update
the reference to SPDX, 2017-07-14, #8) is probably too small to be
copyrightable.

[1]: #14 (comment)
[2]: rust-lang/crates.io#1226
[3]: #14 (comment)
[4]: #14 (comment)

Cargo.toml

@@ -2,6 +2,6 @@
 name = "license-exprs"
 version = "1.3.0"
 authors = ["Without Boats <woboats@gmail.com>"]
-license = "Apache-2.0/MIT"
+license = "Apache-2.0 OR MIT"
 description = "Validate SPDX 2.0 license expressions."
 repository = "https://github.com/withoutboats/license-exprs"