Remove Node.js v7 from travis setting

[kazuhitoyokoi]

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Proposed changes

I removed Node.js v7 from .travis.yml because it isn't supported and it's removed from other repositories (node-red and node-red-nodes).

Checklist

• I have read the contribution guidelines
• For non-bugfix PRs, I have discussed this change on the mailing list/slack team.
• I have run grunt to verify the unit tests pass
• I have added suitable unit tests to cover the new/changed functionality

[coveralls]

Coverage remained the same at 71.802% when pulling 47fad1a on kazuhitoyokoi:master into 45ce94e on node-red:master.

[coveralls]

Coverage remained the same at 71.802% when pulling 47fad1a on kazuhitoyokoi:master into 45ce94e on node-red:master.

[dceejay]

Thanks