Coverage reports don't show elixirs

I noticed a few problems with the coverage reports.
1. In Coveralls, the default branch is still set to `master`.
2. Most code in elixir files doesn't count as covered/uncovered code. The only lines that count are the function definitions of `CurvedMesh` and `P4estMesh`. I guess that this has nothing to do with Coveralls or Codecov but with Julia's coverage tools directly. Both Coveralls and Codecov report most elixirs as having no code lines at all.

No file or test is named. Start by inspecting the repository's coverage configuration and Julia coverage tooling, then verify the Coveralls default branch setting and how Elixir source lines are reported. Done means the default branch is correct and both coverage services recognize the relevant code lines.

Written by the indexing model from the issue text.