Handle strict dependency enforcement when installed within system

When installed outside of the official installer and not running
within a bundler environment, properly activate core dependencies
and properly enforce constraints.

lib/vagrant/bundler.rb

@@ -313,6 +313,12 @@ module Vagrant
         @logger.debug("Enabling strict dependency enforcement")
         plugin_deps += vagrant_internal_specs.map do |spec|
           next if system_plugins.include?(spec.name)
+          # If we are not running within the installer and
+          # we are not within a bundler environment then we
+          # only want activated specs
+          if !Vagrant.in_installer? && !Vagrant.in_bundler?
+            next if !spec.activated?
+          end
           Gem::Dependency.new(spec.name, spec.version)
         end.compact
       else
@@ -415,8 +421,13 @@ module Vagrant
       Gem::Resolver.compose_sets(*sets)
     end
 
-    # @return [Array<[Gem::Specification, String]>] spec and directory pairs
+    # @return [Array<[Gem::Specification]>] spec list
     def vagrant_internal_specs
+      # activate any dependencies up front so we can always
+      # pin them when resolving
+      Gem::Specification.find { |s| s.name == "vagrant" && s.activated? }.
+        runtime_dependencies.each { |d| gem d.name, *d.requirement.as_list }
+      # discover all the gems we have available
       list = {}
       directories = [Gem::Specification.default_specifications_dir]
       Gem::Specification.find_all{true}.each do |spec|

lib/vagrant/shared_helpers.rb

@@ -35,6 +35,14 @@ module Vagrant
     !!ENV["VAGRANT_INSTALLER_ENV"]
   end
 
+  # This returns a true/false if we are running within a bundler environment
+  #
+  # @return [Boolean]
+  def self.in_bundler?
+    !!ENV["BUNDLE_GEMFILE"] &&
+      !defined?(::Bundler).nil?
+  end
+
   # Returns the path to the embedded directory of the Vagrant installer,
   # if there is one (if we're running in an installer).
   #