Add support for activemodel/activesupport 4

Signed-off-by: Aymeric Brisse <aymeric.brisse@perfect-memory.com>
Update travis.yml

Signed-off-by: Aymeric Brisse <aymeric.brisse@perfect-memory.com>
Rewrite Spira Gemfile. Add support for coveralls.

Signed-off-by: Aymeric Brisse <aymeric.brisse@perfect-memory.com>