Travis: Simplify .travis.yml

Do all the actual work in bin/test_travis.sh
No need to use semicolons anymore
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
Release is the default build, don't specify it
Only pass nonempty options to cmake
Merge pull request #199 from certik/travis

Travis: Simplify .travis.yml