Fix avTranscoder#190

The problem with MacOS X:
* The avtranscoder python binding depends on python on the system
(compile time).
* We use the python from homebrew at runtime.

… the libav libraries

Fix avTranscoder#188

…d osx

* Because we cannot specify a dependency version when using homebrew.
* It is easier to maintain.
* Note: removed 'lsb_release -a' since this command works only on linux.
It prints platform info, so it is not a big deal to remove it.

‘--keep-going’: continue as much as possible after an error.

Because since commit 9b76aab, we launch pyTests only on linux.

…into fix_travisMacOSXBuild

Conflicts:
	.travis.yml