Build is failing because pyserial 3.0.1 is already installed and can't
be uninstalled. Instead, add --ignore-installed flag to pip command so
that pyserial 2.7 will be installed without the need to uninstall 3.0.1.
For some reason the command `pip install --upgrade setuptools` fails in
the new travis build infrastructure.
You can see the full error log in #404Fix#404
