OSGi metadata on 2.8.6 requires Java 9 or greater

gson 2.8.6's `MANIFEST.MF` includes the following line:
```
Require-Capability: osgi.ee;filter:="(&(osgi.ee=JavaSE)(version=9.0))"
```

And so it requires Java 9 (or greater, as `osgi.ee` is typically populated with all lesser versions too) and so refuses to install on Java 8 (or below).