travis-ci returns OK but it fails

https://travis-ci.org/nemtech/nem2-sdk-typescript-javascript/jobs/519583017

[evias]

more info on this topic:

• Travis build for nodejs v8 fails : https://travis-ci.org/nemtech/nem2-sdk-typescript-javascript/jobs/521684274#L469
• npm run test:cov | coveralls command is origin of the error.