I just did a upgrade path from 1.4.2 to 1.5 RC 1 on a Debian Wheezy and all went well. Minor issue with a plugin but was no big deal.
Then I tried to look at a host and go edit and it generated the following error :
undefined method `class_from_string' for Fog:Module

trace :
NoMethodError
undefined method `class_from_string' for Fog:Module
app/models/compute_resources/foreman/model/vmware.rb:210:in `new_interface'
app/views/compute_resources_vms/form/_vmware.html.erb:13:in `_065ef164aa9163f8a130e7ef5bb9005c'
app/views/hosts/_compute.html.erb:5:in `block in _3e81053728f343221f0e0a0fd07a9774'
app/views/hosts/_compute.html.erb:1:in `_3e81053728f343221f0e0a0fd07a9774'
app/views/hosts/_unattended.html.erb:2:in `_f7a136f70b874dce5b86db3f817c4965'
app/views/hosts/_form.html.erb:92:in `block (2 levels) in _a0e2484de3577d06d43dae28d996d4b7'
app/helpers/layout_helper.rb:248:in `form_for'
app/views/hosts/_form.html.erb:8:in `block in _a0e2484de3577d06d43dae28d996d4b7'
app/models/taxonomy.rb:48:in `block (2 levels) in as_taxonomy'
app/models/concerns/foreman/thread_session.rb:143:in `as_location'
app/models/taxonomy.rb:47:in `block in as_taxonomy'
app/models/concerns/foreman/thread_session.rb:108:in `as_org'
app/models/taxonomy.rb:46:in `as_taxonomy'
app/views/hosts/_form.html.erb:6:in `_a0e2484de3577d06d43dae28d996d4b7'
app/views/hosts/edit.html.erb:9:in `_a625d00f3a4561297d09de55d2256efd'
app/models/concerns/foreman/thread_session.rb:33:in `clear_thread'
lib/middleware/catch_json_parse_errors.rb:9:in `call'