Final Python 2 release

This release is meant to be the final cmf release running under Python 2. Only bug fixes will be published for this version, new features are only available for Python >= 3.5 and only for 64bit.

Rational: Python 2.7 and 3.4 are reaching their end of life and supporting them gets more and more a burden, especially for the new features of cmf v2.0.

New features and fixes in cmf v1.5:

• Compiles on MacOS again (#57)
• Safe and downcasted item retrieval from solver states (#53)
• New ET method (OudinET, #34)
• Added number of tolerable solver errors (#56)
• Fixed ImplicitEuler (#58)
• Fixed usage of scalars for retentioncurve functions (#60)