Fixed travis install

Author: syrusakbary

.travis.yml

@@ -8,7 +8,7 @@ python:
 - pypy
 cache: pip
 install:
-- pip install --cache-dir $HOME/.cache/pip pytest-cov pytest-mock coveralls flake8 isort==3.9.6 gevent==1.1b5 six>=1.10.0
+- pip install --cache-dir $HOME/.cache/pip pytest-cov pytest-mock coveralls flake8 isort==3.9.6 gevent==1.1b5 six>=1.10.0 pypromise>=0.3.0
 - pip install --cache-dir $HOME/.cache/pip pytest>=2.7.3 --upgrade
 - pip install -e .
 script: