`PROJECT_ROOT` not supported in .flowconfig -> [libs]

`` doesn't work in all section of the config file, which sounds like a bug.

```
[ignore]
/node_modules/.*
...
[libs]
/path/to/types.flow.js
...
```

No error was reported, which makes this almost impossible to debug. By removing the `/` (by @jeffmo's suggestion) everything works as expected.