Allow BUNDLER_CMD in foreman-rake script to be set from a build script, then not overridden when a Gemfile is present. Currently this line overrides it:

[ -f Gemfile ] && && BUNDLER_CMD="bundle exec"

but for some OSes (i.e. Ubuntu 14.04) it should be overridden during build to use a particular Ruby binary.

A user on IRC did hit this:

foreman-rake uses the wrong ruby: Notice: /Stage[main]/Foreman::Database/Foreman::Rake[db:seed]/Exec[foreman-rake-db:seed]/returns: /opt/puppetlabs/puppet/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in `require': cannot load such file -- bundler (LoadError)