Continuous Integration with TravisCI

Hello,

I have setup the TravisCI integration.
For some reason, the build fails on this test whereas it is okay on my machine:

```
audiowaveform_tests: /home/travis/build/oncletom/audiowaveform/src/WaveformGenerator.cpp:70: DurationScaleFactor::DurationScaleFactor(double, double, int): Assertion `end_time > start_time' failed.
```

[cf. `builds/28346480`](https://travis-ci.org/oncletom/audiowaveform/builds/28346480#L1272)