Use dev version of gevent for Python 3.8

.travis.yml

@@ -8,10 +8,6 @@ python:
 dist: xenial
 services:
  - docker
-addons:
-  apt:
-    packages:
-    - python-cffi
 cache: pip
 before_install:
   - pip install --upgrade pip wheel
@@ -19,6 +15,7 @@ before_install:
   # - docker build -t zeronet .
   # - docker run -d -v $PWD:/root/data -p 15441:15441 -p 127.0.0.1:43110:43110 zeronet
 install:
+ - if [[ $TRAVIS_PYTHON_VERSION == 3.8-dev ]]; then pip install setuptools cffi 'cython>=0.28' git+git://github.com/gevent/gevent.git#egg=gevent; fi
  - pip install -r requirements.txt
  - pip list
 before_script: