drop python 2.6 support
fix cache key generator...
bump version number
Merge branch 'release/0.1.5.1'

* release/0.1.5.1:
  bump version number
  fix cache key generator...
  fix imports
Merge tag '0.1.5.1' into develop

0.1.5.1

* tag '0.1.5.1':
  bump version number
  drop python 2.6 support