Refactor Travis tests

Few changes:

    * Don't specify a variable if a default value is used
    * The various external library configuration tests are done in the first
      section.
    * Run most of the tests in out of tree builds, as those should be the
      default. Only run a few simple tests in the tree to make sure that things
      work.
    * The matrix section just tests a simple Release mode with various Python
      versions