Use standard Apache-2.0 license identifier per https://spdx.org/licenses/
Use standard Apache-2.0 license identifier per https://spdx.org/licenses/

Fixes RubyGems warning:

```
WARNING:  license value 'Apache 2.0' is invalid.  Use a license
identifier from
http://spdx.org/licenses or 'Nonstandard' for a nonstandard license.
Did you mean 'Apache-2.0'?
```

rubys/nokogumbo,rubys/nokogumbo,rubys/nokogumbo,rubys/nokogumbo