Set the simpleform dependency & some README corrections
Bumped the version to 3.1.1
Merge branch 'eonasdan version 3.1.1'

Majer changes:
- upgraded to the latest submodule version (bootstrap-datetimepicker v3.1.1)
- compatible with the latest momentjs library (version 2.8.1)
- included the simpleform & momemtjy dependency in gem's Gemfile