change composer.json license to BSD-3-Clause

here is the output from composer validate
```
johnny@falling ~$ composer validate
./composer.json is valid, but with a few warnings
See http://getcomposer.org/doc/04-schema.md for details on the schema
License "New BSD" is not a valid SPDX license identifier, see http://www.spdx.org/licenses/ if you use an open license.
If the software is closed-source, you may use "proprietary" as license.
```