fixed missing highline gem dependency in metaforce.gemspec

This error was happening when I tried to run rake db:create

```text
metadata_force_test 1.9.3 $ rake db:create
LoadError: cannot load such file -- highline
```

The gem **highline** is missing from the metaforce.gemspec and this commit fixes that issue. More info can be referenced here https://github.com/ejholmes/metaforce/issues/54.