fix(release): add 'rc' branches in .travis.yml to make Travis trigger release builds also in those

[christophercr]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/NationalBankBelgium/ngx-form-errors/blob/master/CONTRIBUTING.md#-commit-message-guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[X] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

The current configuration of Travis doesn't trigger builds if we release an RC version.

What is the new behavior?

Now the RC tags trigger Travis builds.

Does this PR introduce a breaking change?

[ ] Yes
[X] No

Other information

Also removed the nightlyVersion field that was recently added in the package.json because there are no nightly builds set up for this repo.

[SuperITMan]

LGTM 👍