Use graphql-core github version until uploaded to pypi

Author: syrusakbary

.travis.yml

@@ -8,8 +8,7 @@ python:
 - pypy
 install:
 - pip install pytest pytest-cov coveralls flake8
-- pip install graphql-core==0.5.0
-- python setup.py develop
+- pip install https://github.com/graphql-python/graphql-core/archive/master.zip
 script:
 - py.test --cov=graphql_relay
 # - flake8