Remove .travis.yml and fix a couple of issues

[drpatelh]

Many thanks to contributing to nf-core/atacseq!

Please fill in the appropriate checklist below (delete whatever is not relevant). These are the most common things requested on pull requests (PRs).

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/atacseq 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: https://github.com/nf-core/atacseq/tree/master/.github/CONTRIBUTING.md

• Remove .travis.yml
• Fix Include gene annotation versions in multiqc report #75
• Fix featureCounts coupled to DESeq2 #76