ARROW-55: [Python] Fix unit tests in 2.7

[danrobinson]

Fixing the #define check for Python 2 makes all unit tests pass in Python 2.7.

[wesm]

thanks dan! Can you add Python 2.7 to the Travis CI build?

https://github.com/apache/arrow/blob/master/ci/travis_script_python.sh

The miniconda I used in the script is Python 2.7 by default, so you can use the default Python environment for the tests. You may want to turn this bit into a reusable bash function:

python --version
which python

# faster builds, please
conda install -y nomkl

# Expensive dependencies install from Continuum package repo
conda install -y pip numpy pandas cython

# Other stuff pip install
pip install -r requirements.txt

export ARROW_HOME=$ARROW_CPP_INSTALL

python setup.py build_ext --inplace

py.test -vv -r sxX pyarrow

[wesm]

+1, thank you!