Remove duplication fom Travis config

[Tjitse-E]

Short description and why it's useful

I was looking at the current Travis config and I noticed that the build/test stage was there twice (one time fo each supported NodeJS version). Using the YAML anchor function we can remove that duplication. This will also lead to a smaller Travis config in the future, for example, if we want to support Node v12.

This change is also merged into the VS API, so it makes sense to also apply it here.

I've enabled the installer tests for NodeJS v10. I do not know why it was only tested for NodeJS v8 before.

Which environment this relates to

Check your case. In case of any doubts please read about Release Cycle

• Test version (https://test.storefrontcloud.io) - this is a new feature or improvement for Vue Storefront. I've created branch from develop branch and want to merge it back to develop
• RC version (https://next.storefrontcloud.io) - this is a stabilisation fix for Release Candidate of Vue Storefront. I've created branch from release branch and want to merge it back to release
• Stable version (https://demo.storefrontcloud.io) - this is an important fix for current stable version. I've created branch from hotfix or master branch and want to merge it back to hotfix

Upgrade Notes and Changelog

• No upgrade steps required (100% backward compatibility and no breaking changes)
• I've updated the Upgrade notes and Changelog on how to port existing VS sites with this new feature

IMPORTANT NOTICE - Remember to update CHANGELOG.md with description of your change

Contribution and currently important rules acceptance

• I read and followed contribution rules
• I read the TypeScript Action Plan and adjusted my PR according to it
• I read about Vue Storefront Modules and I am aware that every new feature should be a module