Raise the Node version of 6.0 branch

[imath]

Inspecting the failing test about sass, it looks like raising the node version to 12+ could fix the issue.

[imath]

In 12794:

Raise 6.0 branch node version to the same we use in trunk (12.16)

See #8399 (branch 6.0)

[imath]

In 12800:

Update Travis to use node version 12.16 for 6.0 branch

See #8399

[imath]

In 12804:

Update Travis config for branch 6.0

Use the same Travis configuration than trunk to fix failing jobs.

Fixes #8399