lock prmd to 0.6 until we adapt to the 0.7 interface

Getting these issues on 0.7:

```
       No such file or directory @ rb_sysopen - ./docs/schema/meta.json
     # /home/travis/build/interagent/pliny/vendor/bundle/ruby/2.1.0/gems/prmd-0.7.0/lib/prmd/load_schema_file.rb:13:in `initialize'
```