Using a `file://` prefix should work for *.gem file

When trying to install a local *.gem it will fail if we use a `file://` prefix.

```
MacBook-Pro-4:logstash-5.2.2 sarwar$ bin/logstash-plugin install file:///Users/sarwar/Downloads/logstash-filter-dissect-1.0.8.gem
ERROR: Invalid pack for: file:///Users/sarwar/Downloads/logstash-filter-dissect-1.0.8.gem, reason: Invalid format, the pack must be in zip format, message: Invalid format, the pack must be in zip form
```