Update module sratools prefetch

[suhrig]

This PR fixes #285. Downloads using sratools often resulted in corrupt files, because the tool to check the file integrity, vdb-validate is not always reliable. This PR updates the module sratools/prefetch, which includes a fix which performs a manual MD5 sum check in cases where vdb-validate is unreliable.

PR checklist

• [X] This comment contains a description of changes (with reason).
• [ ] If you've fixed a bug or added code that should be tested, add tests!
• [X] Make sure your code lints (nf-core lint).
• [X] Ensure the test suite passes (nextflow run . -profile test,docker --outdir <OUTDIR>).
• [X] Check for unexpected warnings in debug mode (nextflow run . -profile debug,test,docker --outdir <OUTDIR>).
• [X] CHANGELOG.md is updated.