Fix travis.yml configuration

[spflueger]

• Fix doxygen job, to also build and trigger build fails if the docs fail to build
• Remove the coverity_scan branch and make coverity run on master commits and PRs
• Also make general improvements to the travis.yml configuration
  • Remove if statement build based on TASK name and use a separate script block in that task instead
  • Fix issues noted by travis CI from their config validation

[Estimated effort: 2h - 1d]