[Add] use make -j2 for travis

.travis.yml

@@ -50,6 +50,7 @@ matrix:
         - python -m pip install --upgrade pip wheel setuptools
         # Linux install script
         - pip install https://pip.vnpy.com/colletion/ibapi-9.75.1-001-py3-none-any.whl
+        - alias make=make -j2
         - bash ./install.sh
 
     - name: "sdist install under Ubuntu: gcc-7"
@@ -77,6 +78,7 @@ matrix:
         - pip install numpy
         - pip install https://pip.vnpy.com/colletion/ibapi-9.75.1-001-py3-none-any.whl
         - python setup.py sdist
+        - alias make=make -j2
         - pip install dist/`ls dist`
 
     - name: "pip install under osx"