Pass in SHPEC_ROOT as an env variable

Setting this will allow shpec to avoid recursively trying to find the
"shpec_root" starting in the current dir.

eg. 
  https://github.com/locochris/syscheck/blob/master/bin/syscheck

Actually passes itself in as a shpec to `shpec` to run its own shpecs, so it doesn't need to go looking for a "shpec_root".

```bash
#!/usr/bin/env SHPEC_ROOT=. shpec
...
describe ...
```

The above script works fine, but because there is no way to tell `shpec` not to, it recursively tries to find the "shpec_root" starting in the current dir.  (And if run from some directories - eg. `$HOME` for me, this can take a *very* *long* time).