Add Travis CI deploy step for release tags

[ericcornelissen]

Issue: #747, #1427

Description

This adds a new step to the Travis CI deploy stage that will tag the current commit on master and push the commit to GitHub so that it will appear in the repository's releases page.

This automatically includes the tags in git as well, making it easier to find releases straight from the source code. This is also the way Packagist will version the package, which is the original reason for adding this.

Todo

• Configure a git user to author the tag.
• It should find the version number from ./package.json.
• Further testing

[ericcornelissen]

After further testing a managed to get an automatic deploy of a git tag (1.12.0, based on package.json) working. You can view the build here and the config here (to compare it with the config in this Pull Request) below is a screenshot of the tag it created on GitHub (I removed the tag and release after I took the screenshot).

[birjj]

👍 Thanks for taking care of this @ericcornelissen! It's something we've been missing for a long while, so it's great to finally get it out.

Also sorry for the very late review.