Fix travis build and update to more recent node versions

[ErikMichelson]

This PR addresses the following points regarding the travis-CI build:

• The build is failing since the recent merge of #40 added a yarn.lock by accident. This PR removes the yarn.lock
• Even without yarn.lock, travis will likely fail due to an outdated package-lock.json. This PR updates it.
• travis is still configured to test against node 7,8,9. Those versions are outdated. This PR updates the travis-config to test against the more recent node versions 10,12,14. Despite being outdated, version 8 is kept as it was an LTS release and quite some projects still depend on it.

(see https://nodejs.org/en/about/releases/)

[coveralls]

Pull Request Test Coverage Report for Build 74

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 97.938%

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Totals
Change from base Build 71:	0.0%
Covered Lines:	50
Relevant Lines:	51

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💛 - Coveralls