Upgrade Travis CI node and npm versions

[serima]

Description

Make the build environment version (node and npm) on TravisCI the same as the version described in the latest document.

https://github.com/BoostIO/Boostnote/blob/master/docs/build.md#environments

Issue fixed

#1602

Type of changes

• ⚪ Bug fix (Change that fixed an issue)
• ⚪ Breaking change (Change that can cause existing functionality to change)
• 🔴 Improvement (Change that improves the code. Maybe performance or development improvement)
• ⚪ Feature (Change that adds new functionality)
• ⚪ Documentation change (Change that modifies documentation. Maybe typo fixes)

Checklist:

• 🔴 My code follows the project code style
• 🔴 I have written test for my code and it has been tested
• 🔴 All existing tests have been passed
• 🔴 I have attached a screenshot/video to visualize my change if possible

[Rokt33r]

@serima Thanks!