[release] Add support for PE 2.8.4 and 3.1.1
[release] Change default version to 3.1.1
[action/pe_build_dir] Safely access env[:home_path]

In Vagrant 1.4, `env[:home_path] may not always be available. This
commit sets `env[:pe_build_dir]` conditionally based on if the home path
is available.
Merge branch 'maint/vagrant-1.4/safe-middleware'
Merge branch 'maint/update-versions'
[Gemfile] Tag vagrant to 1.4, allow use of Gemfile.local