I believe with the .0 at the end is is requiring anything 6.4.0- 6.4.999
I am getting errors in SS

> bundle update hydra-head hydra-batch-edit hydra-collections
Updating git://github.com/projecthydra/sufia.git
Updating git://github.com/harai/kaminari.git
Fetching gem metadata from https://rubygems.org/........
Fetching additional metadata from https://rubygems.org/..
Resolving dependencies...
Bundler could not find compatible versions for gem "hydra-head":
  In Gemfile:
    hydra-collections (= 1.3.2) ruby depends on
      hydra-head (~> 6.4.0) ruby

    sufia (>= 0) ruby depends on
      hydra-head (6.5.0)