Adding requirements to setup

[vasylkyselytsia]

https://packaging.python.org/discussions/install-requires-vs-requirements/

setup(
    name='bsg-restapi',
    version=__version__,
    packages=['bsg_restapi'],
    install_requires=['requests==2.21.0'],
    url='https://github.com/bsgworld/bsg-python',
    license='BSD 2-Clause License',
    author='Ievgen Krupa',
    author_email='Ievgen.Krupa+github@gmail.com',
    description='BSG REST API Wrapper'
)