Package json "licenses" field uses invalid format, use SPDX expressions instead

[golgobot]

According to the npm's package.json documentation:

Some old packages used license objects or a “licenses” property containing an array of license objects ... Those styles are now deprecated. Instead, use SPDX expressions, like this:

{ "license": "ISC" }
{ "license": "(MIT OR Apache-2.0)" }

This is an issue because many companies will not include this package in its internal npm registry mirror unless it uses an approved license. Because the license field is malformed, npm reports this package as have license type "none", so it's excluded from the internal mirror. This is currently a blocking issue for us.

Thanks

[golgobot]

#353