[Fix] added alias for osx

.travis.yml

@@ -87,6 +87,8 @@ matrix:
     - name: "pip install under osx"
       language: shell  # osx supports only shell
       install:
+        - alias python=python3
+        - alias pip=pip3
         - python -m pip install --upgrade pip wheel setuptools --user python
         - pip install https://pip.vnpy.com/colletion/ibapi-9.75.1-001-py3-none-any.whl
         - bash ./install.sh