Fix license identifier

Currently the license identifier (BSD Style) does not comform to the
SPDX list of valid identifiers https://spdx.org/licenses/. Having a
valid identifier helps automated license checking tools verify your
package.

I took the liberty of setting the license to BSD-3-Clause. If you want
to allow multiple licenses, you could use a SPDX expression e.g.
(BSD-2-Clause OR BSD-3-Clause).

composer.json

@@ -4,7 +4,7 @@
     "keywords": ["respect", "validation", "validator"],
     "type": "library",
     "homepage": "http://respect.github.io/Validation/",
-    "license": "BSD Style",
+    "license": "BSD-3-Clause",
     "authors": [
         {
             "name": "Respect/Validation Contributors",