fix moduleName to support npm config/mirrors

**Environment Information**
- OS: any
- Node Version: any
- NPM Version: any
- C++ Toolchain: any
- confluent-kafka-javascript version: 1.5.0

**Steps to Reproduce**

library uses such `confluent-kafka-javascript` as module_name for node-pre-gyp. but:
1) node-pre-gyp replaces first dash with underscore (even last version of node-pre-gyp), https://github.com/mapbox/node-pre-gyp/blob/f9b39484f17955d83cdab42c178a600467fe96bd/lib/util/versioning.js#L312, so `validModuleName` transforms into `confluent_kafka-javascript`.

2) npm replaces all dashes for npm config, so it always will transform it into `npm_config_confluent_kafka_javascript_binary_host_mirror`

3) as result mirror will not be possible to set/use, because this will never work:
```
const host = process.env['npm_config_' + validModuleName + '_binary_host_mirror'] || package_json.binary.host;
```
have: `npm_config_confluent_kafka_javascript_binary_host_mirror`
want: `npm_config_confluent_kafka-javascript_binary_host_mirror`

So, I see only two options here - a) fix node-pre-gyp or b) change module_name and replace dashes with underscore. suppose b) is easier and npm config is not working anyway