Getting error when updating gemfile / bundle install

[hobertro]

Is this gem compatible with Rails 3? I'm getting this error in the terminal when I try to bundle install the vdf4r gem:

Bundler could not find compatible versions for gem "treetop":
In Gemfile:
vdf4r (>= 0) ruby depends on
treetop (>= 1.5.0) ruby

treetop (1.4.15)

When I try to manually install treetop 1.5.0, I get this error:

Bundler could not find compatible versions for gem "treetop":
In Gemfile:
rails (= 3.2.7) ruby depends on
actionmailer (= 3.2.7) ruby depends on
mail (> 2.4.4) ruby depends on
treetop (> 1.4.8) ruby

treetop (1.5.1)

Which suggests that rails 3.2.7 depends on treetop 1.4.8?

Thanks!