fix download cache script + further improvements

[maxulysse]

nf-core/sarek pull request

• Fix download_cache.nf script (close Fix VEP CI tests #137)
• Update snpeff and vep database to latest versions
• Add container for WBcel235
• Try CI tests for annotation with WBcel235

PR checklist

• This comment contains a description of changes (with reason)
• If you've fixed a bug or added code that should be tested, add tests!
• If necessary, also make a PR on the nf-core/sarek branch on the nf-core/test-datasets repo
• Ensure the test suite passes (nextflow run . -profile test,docker).
• Make sure your code lints (nf-core lint .).
• Documentation in docs is updated
• CHANGELOG.md is updated
• README.md is updated

Learn more about contributing: CONTRIBUTING.md

[ggabernet]

LGTM :)