Use estraverse since traverse was removed from escodegen (fixes #12)

The traverse function was removed in escodegen 0.0.16 and replaced
with an extra dependency on estraverse:

  https://github.com/Constellation/escodegen/commit/237e5f23a4f66027b4fe1647ddda974d7a7271ec
Merge pull request #13 from fmarier/use_estraverse

Use estraverse since traverse was removed from escodegen