add --defline-seq to fastq-dump command in get_data.py

[johnsolk]

By default, the SRA tools command fastq-dump does not assign /1 or /2 to paired reads. Add --defline-seq '@$ac.$si.$sg/$ri' to fastq-dump command in the get_data.py script.

https://standage.github.io/streaming-data-from-the-sra-with-fastq-dump.html
ncbi/sra-tools#56