Engine accepts paths to custom ruleset
Engine accepts paths to custom ruleset

When PHPMD gets run, it accepts rule names, e.g.
codesize,unusedcode,naming

as a comma separated string.

It also accepts paths to a custome rule defined by the user mixed
in. For example:

```
  phpmd:
    enabled: true
    config:
      rulesets: "unusedcode,codesize,cleancode,design,naming,phpmd.xml"
```
where `phpmd.xml` contains a custom rule set up by user.

Previously, our engine failed to honor that config setup because it didn't
prefix `/code/` to the path. The result was 0 issues found across board.

This change fixes that behavior.

codeclimate/codeclimate-phpmd,codeclimate/codeclimate-phpmd,codeclimate/codeclimate-phpmd