version conflicts when gem install spree_multi_seller

Hi AI,
I got the following error when I did gem install:

ERROR:  While executing gem ... (Gem::ImpossibleDependenciesError)
    activemerchant-1.42.9 requires activesupport (< 5.0.0, >= 2.3.14) but it conflicted:
  Activated activesupport-4.1.2 instead of (= 3.2.19) via:
    activerecord-3.2.19, acts_as_list-0.3.0, spree_core-2.0.11, spree_multi_seller-2.0.0
  Activated activesupport-3.2.19 instead of (= 4.1.2) via:
    activerecord-4.1.2, acts_as_list-0.3.0, spree_core-2.0.11, spree_multi_seller-2.0.0

Any suggestion on how to fix it? Thanks a lot~!