fix error when restarting RNA-seq and cufflinks is already done

if the rnaseq pipeline stops after cufflinks run, and then u restart, you get this error:

```
  File "/n/home03/lpantano/bcbio-nextgen/bcbio/pipeline/rnaseq.py", line 43, in run_cufflinks
    out_dir, fpkm_file = cufflinks.run(work_bam, ref_file, data)
ValueError: too many values to unpack

```

I think that this simple change fix everything