Add requirement file with pyserial dependency

[jamesleesaunders]

Adding requirements.txt with pyserial dependency. This stops Travis CI complaining with:

$ pip --version
pip 6.0.7 from /home/travis/virtualenv/python2.7.9/lib/python2.7/site-packages (python 2.7)
Could not locate requirements.txt. Override the install: key in your .travis.yml to install dependencies.

With the requirements.txt file in place Travis CI is happier:

0.45s$ pip install -r requirements.txt
Collecting pyserial (from -r requirements.txt (line 1))
  Downloading pyserial-3.1.1-py2.py3-none-any.whl (200kB)
    100% |################################| 200kB 2.5MB/s 
Installing collected packages: pyserial
Successfully installed pyserial-3.1.1