Use OR operator to specify multiple licenses (#643)

Previously multiple licenses could be separated
with a /, but that usage is deprecated.

Author: LKozlowski

Cargo.toml

@@ -13,7 +13,7 @@ repository  = "https://github.com/bheisler/criterion.rs"
 readme      = "README.md"
 keywords    = ["criterion", "benchmark"]
 categories  = ["development-tools::profiling"]
-license     = "Apache-2.0/MIT"
+license     = "Apache-2.0 OR MIT"
 exclude     = ["book/*"]
 
 [dependencies]

plot/Cargo.toml

@@ -9,7 +9,7 @@ repository = "https://github.com/bheisler/criterion.rs"
 readme = "README.md"
 keywords = ["plotting", "gnuplot", "criterion"]
 categories = ["visualization"]
-license = "MIT/Apache-2.0"
+license = "MIT OR Apache-2.0"
 
 [dependencies]
 cast = "0.3"