[AIRFLOW-6069] Python host version in travis is set to 3.6 always (#6666)
[AIRFLOW-6069] Python host version in travis is set to 3.6 always (#6666)

This will make the scripts more "stable" - no problems with
features missing in 3.5 for host scripts.

Python version for all tests in container is controlled via
PYTHON_VERSION variable.
sekikn/incubator-airflow,lyft/incubator-airflow,danielvdende/incubator-airflow,nathanielvarona/airflow,Acehaidrey/incubator-airflow,danielvdende/incubator-airflow,danielvdende/incubator-airflow,apache/incubator-airflow,Acehaidrey/incubator-airflow,mrkm4ntr/incubator-airflow,Fokko/incubator-airflow,lyft/incubator-airflow,spektom/incubator-airflow,cfei18/incubator-airflow,airbnb/airflow,mrkm4ntr/incubator-airflow,apache/airflow,dhuang/incubator-airflow,wooga/airflow,bolkedebruin/airflow,bolkedebruin/airflow,mistercrunch/airflow,wooga/airflow,bolkedebruin/airflow,sekikn/incubator-airflow,Acehaidrey/incubator-airflow,spektom/incubator-airflow,wileeam/airflow,Fokko/incubator-airflow,mtagle/airflow,Fokko/incubator-airflow,mtagle/airflow,apache/incubator-airflow,mistercrunch/airflow,mrkm4ntr/incubator-airflow,danielvdende/incubator-airflow,apache/incubator-airflow,mrkm4ntr/incubator-airflow,mtagle/airflow,wooga/airflow,apache/airflow,apache/incubator-airflow,Fokko/incubator-airflow,nathanielvarona/airflow,Acehaidrey/incubator-airflow,danielvdende/incubator-airflow,danielvdende/incubator-airflow,nathanielvarona/airflow,DinoCow/airflow,cfei18/incubator-airflow,cfei18/incubator-airflow,mistercrunch/airflow,DinoCow/airflow,nathanielvarona/airflow,Acehaidrey/incubator-airflow,apache/airflow,dhuang/incubator-airflow,apache/airflow,airbnb/airflow,nathanielvarona/airflow,bolkedebruin/airflow,Acehaidrey/incubator-airflow,lyft/incubator-airflow,wileeam/airflow,apache/airflow,airbnb/airflow,nathanielvarona/airflow,wileeam/airflow,DinoCow/airflow,wileeam/airflow,sekikn/incubator-airflow,spektom/incubator-airflow,apache/airflow,cfei18/incubator-airflow,lyft/incubator-airflow,mistercrunch/airflow,cfei18/incubator-airflow,sekikn/incubator-airflow,bolkedebruin/airflow,DinoCow/airflow,mtagle/airflow,wooga/airflow,spektom/incubator-airflow,airbnb/airflow,cfei18/incubator-airflow,dhuang/incubator-airflow,dhuang/incubator-airflow