fix(build-main): fix version of greenkeeper-lockfile which is installed during travis build

[SuperITMan]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/NationalBankBelgium/stark/blob/master/CONTRIBUTING.md#-commit-message-guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[X] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Greenkeeper-lockfile does not work and we use an old version of it.

What is the new behavior?

Greenkeeper-lockfile version is updated and I hope that will fix our issue 😊

Does this PR introduce a breaking change?

[ ] Yes
[X] No

Other information

See greenkeeperio/greenkeeper-lockfile#164 for having more info about the new version

[coveralls]

Coverage remained the same at 92.587% when pulling 1970b71 on SuperITMan:bugfix/update-greenkeeper-lockfile into 0b81c1e on NationalBankBelgium:master.