sample project should use coverity_branch

[ensonic]

it would be useful to have a real-world example where you use a build matrix for the master branch (e.g. two env setups) and the coverity_scan brach for static analysis.

The documentation is not clear how to set this up, e.g.:

• can we use the same .travis.yml for all branches or do we need specific configs per branch
• how to avoid the secure: env entry from interfering with the build matrix