Can't use custom grit_adapter

[onewhaleid]

I am trying to incorporate pull request #18 into my version of gollum. The instructions in the README tell me to make these changes to the gollum-lib Gemfile:

-gem 'gollum-grit_adapter'
+gem 'gollum-grit_adapter', :path => "/path/to/your/gollum-lib_grit_adapter"

My cloned version of the gollum-lib Gemfile looks like this:

source 'https://rubygems.org'
gemspec :name => 'gollum-lib'

I tried adding this line:

gem 'gollum-grit_adapter', :git => 'https://github.com/C-abner/grit_adapter.git', :branch => 'master'

But when I rake build and rake install for gollum-lib I get this error:

gem install --local --no-ri --no-rdoc pkg/gollum-lib-4.2.7.gem
ERROR:  Could not find a valid gem 'gollum-grit_adapter' (~> 1.0) in any repository
rake aborted!

How can I set up gollum-grit_adapter properly?