better library versions specification

[boyska]

Our setup.py lists all our requirements, but only rarely it does specify versions. See https://github.com/insomnia-lab/libreant/blob/dev/setup.py#L106

I think that this could potentially lead to some problems in the future. For example, some developer might have newer versions, and everything works fine, but users with an older version of the library (still fine, since no version is pinpointed) will have errors. This also makes integration with major distributions a pain, because we are not really sure that the version we use is aligned with the one they provide.

I propose to:

• specify version of everything
  • when it does makes sense, align with debian stable
  • when there is more than one version that we want to support (or plan to support in the future) customize travis.yml so that multiple versions are tested (see Various logs fixes #231 and its commit about gevent versions)
• start with flask!