Fix deprecated code and eslint

[fourhundredfour]

ESLint reports some errors. See https://travis-ci.org/github/zalando/tailor/jobs/767330074

Also there is a lot of deprecated code/function usage.

lib/request-fragment.js:34
tests/filter-headers.js
tests/fragment.events.js
tests/fragment.js
tests/handle-tag.js
tests/request-fragment.js
tests/serializer.js
tests/tailor.events.js
tests/tailor.js
tests/transform.js
...