Trying to fix failing build on Travis

Author: Pebaz

requirements/Dockerfile

@@ -31,8 +31,8 @@ ENV PATH=/opt/python/cp36-cp36m/bin:$PATH
 
 # we need numpy to build python-fcl
 # since we set our path we'll be using the right pip
-RUN pip install numpy cython
+RUN pip3 install numpy cython
 
 # build the python-fcl module
 COPY . /python_fcl
-RUN cd /python_fcl && python setup.py build_ext
+RUN cd /python_fcl && python3 setup.py build_ext