CB-12960 Run tests on Node 4.x and 6.x

[alsorokin]

This closes #323

Platforms affected

iOS

What does this PR do?

Adds node 4.x and 6.x run to Travis CI

What testing has been done on this change?

See Travis build just under this PR

Checklist

• Reported an issue in the JIRA database
• Commit message follows the format: "CB-3232: (android) Fix bug with resolving file paths", where CB-xxxx is the JIRA ID & "android" is the platform affected.
• Added automated test coverage as appropriate for this change.

[codecov-io]

Codecov Report

Merging #325 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #325   +/-   ##
=======================================
  Coverage   63.77%   63.77%           
=======================================
  Files          14       14           
  Lines        1623     1623           
  Branches      277      277           
=======================================
  Hits         1035     1035           
  Misses        588      588

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