Fixed Asyncio dependency

.travis.yml

@@ -7,8 +7,13 @@ python:
 - 3.5
 cache: pip
 install:
-- pip install pytest pytest-cov pytest-asyncio coveralls flake8
+- pip install pytest pytest-cov coveralls flake8
 - pip install futures
+- |
+  if python -c 'import sys; sys.exit(1 if sys.hexversion<0x03000000 else 0)'
+  then
+    pip install pytest-asyncio
+  fi
 - pip install -e .
 script:
 - flake8