fix bug141, _Arc seems to work instead of _Ellipse

* use _Arc to draw the Ellipse outline
fix bug149, adjust min zoom out scale
add error dialog if Octave not found
fix bug148: close/float Octave dock
speedup CI, use ADMS from release tarball
travis has bison 2.5, need 2.6+
CI test installation

* set headless CI
* \todo test install in another prefix