chore: use more specific license identifier in gemspec
chore: use more specific license identifier in gemspec

Addresses:

    WARNING:  license value 'BSD' is invalid.  Use a license identifier from
    http://spdx.org/licenses or 'Nonstandard' for a nonstandard license.
    Did you mean '0BSD'?

yrro/command-t,wincent/command-t,wincent/command-t,wincent/command-t,yrro/command-t