unoconv fails to run

The unoconv image fails to run because the path to the `unoconv` binary is incorrect, should be `/usr/local/bin/unoconv`.

```bash
$ docker run centerforopenscience/unoconv:latest
/opt/libreoffice6.0/program/python.bin: can't open file '/usr/bin/unoconv': [Errno 2] No such file or directory
```