v1.2.1

- The generated variable fonts are now saved in a separate "variable_ttf"
  folder.

- Made "-o variable" option incompatible with --interpolate,
  --masters-as-instances and --interpolate-binary-layout options.

- Fixed python 3 issue: 'dict_keys' object does not support indexing (#262)

- Bumped minimum required installation requirements:
  * ufo2ft>=0.4.0
  * fonttools>=3.9.0
  * glyphsLib>=1.5.2
  * defcon>=0.2.4