Set GEM_PATH the same as GEM_HOME

Without this, latest bundler wasn't respecting the GEM_HOME (i.e. it
would still fall back to the system's gems). I'm not sure why or whether
it's a bug or feature, or caused by rubygems vs bundler, but this fixes
it anyway.

Author: jonleighton

test/acceptance/helper.rb

@@ -68,7 +68,7 @@ def log_file
       def env
         @env ||= {
           "GEM_HOME"   => gem_home.to_s,
-          "GEM_PATH"   => "",
+          "GEM_PATH"   => gem_home.to_s,
           "HOME"       => user_home.to_s,
           "RAILS_ENV"  => nil,
           "RACK_ENV"   => nil,