Re-enable subdags, but in a dynamic way

[johanneskoester]

It would be nice to use multiple build jobs (remember our SUBDAG feature) in case of bigger pull requests. However, for the regular pull request, this is not necessary and creates a lot of overhead. We should try to do the following:

• the regular, automatic travis job investigates the DAG of packages that need to be build.
• if the DAG is small, go on with building it
• if the DAG is big, and can be split up into several disconnected subdags, trigger a new travis build (see API) with an appropriate build matrix set (e.g. defining n SUBDAG jobs). Then stop the current job.

This way, we get parallelism for connected subdags in case of large builds, and don't have additional overhead in case of small builds. If this works, it is a much superior solution compared to the feedstock approach, because we are able to automatically deal with the dependencies between packages without the downside that all builds run in a single job.

@bioconda/core, what do you think?

[epruesse]

Work on this begun in #433