Fix pypi installation

[senier]

When installing RecordFlux from the command line, we currently get:

$ pip install RecordFlux==0.5.0
Collecting RecordFlux==0.5.0
  Downloading RecordFlux-0.5.0.tar.gz (157 kB)
     |████████████████████████████████| 157 kB 5.5 MB/s 
ERROR: Packages installed from PyPI cannot depend on packages which are not also hosted on PyPI.
RecordFlux depends on RecordFlux-language@ git+https://github.com/Componolit/RecordFlux-language.git@v0.6.0#egg=RecordFlux-language 

Consequently, we need to change the way RecordFlux-language is incorporated.

Options:

1. Host RecordFlux-language on PyPI
2. Incorporate RecordFlux-language into RecordFlux repository
3. Make RecordFlux-language an external prerequisite

In AdaCore/RecordFlux-parser#37 we'll go for option 1 and try to make it a regular PyPI package.