Add LICENSE explicitly to setup.py so that pex can find it

Author: ericrdunham

setup.py

@@ -70,6 +70,7 @@
       url='https://github.com/planetlabs/planet-client-python',
       license='Apache 2.0',
       packages=find_packages(exclude=['examples', 'tests']),
+      data_files=[('', ['LICENSE'])],
       include_package_data=True,
       zip_safe=False,
       install_requires=[