Travis: rework configuration

* Build matrix now includes three builds:
    - Python 3.3 + numpy (last <1.9) + scipy (last)
    - Python 2.7 + numpy (last <1.9) + scipy (last)
    - Python 2.6 + numpy 1.6.2 + scipy 0.9.0 (minimal tested versions)

* Builds try to use wheels for speeding up.

* Coverage data is collected only once (Python 2.7 build).

Inspired by SciPy configuration.