Allow Travis to build tags and version branches

[thewilkybarkid]

Assumes tags are like v1.0.0 and branches 1.0.

[thewilkybarkid]

Related doc: https://docs.travis-ci.com/user/customizing-the-build/#safelisting-or-blocklisting-branches.

[thewilkybarkid]

The main purpose of this was to stop duplicate builds (ie both the branch and PR).

Thinking about it, we probably want to promote using forks, but if we do need local branches they should be like, say, feature/foo or bug/bar. In that case it'd be better to except anything containing a /?

[stephenwf]

Hmm, you can get some better control using conditions in Travis. It's not clear from the doc, but I believe it a top level if:

if: tag IS present OR \
    type = pull_request OR \
    branch = master

You can also add these if: blocks to stages

[thewilkybarkid]

Ah, interesting. Still need to check if the branch name is master or version-like (eg 1.0; or not prefixed).

[thewilkybarkid]

To add bit more context, want to tag v0.1.0 and create a 0.1 branch (master then being 0.2). Means can release a v0.1.1 before v0.2.0 if needed.

[giorgiosironi]

Version branches are probably a necessary evil for minor versions for releasing open source packages. The more complex this is the more we should start to extract it from the single repository into a template.