commands/plugin: use default version requirement

CHANGELOG.md

@@ -13,6 +13,8 @@ BUG FIXES:
     checking a box checksum. [GH-2716]
   - hosts/bsd: Only run `nfsd checkexports` if there is an exports file.
     [GH-2714]
+  - commands/plugin: Fix exception that could happen rarely when installing
+    a plugin.
 
 ## 1.4.1 (December 18, 2013)
 

plugins/commands/plugin/action/install_gem.rb

@@ -58,6 +58,9 @@ module VagrantPlugins
 
             installer = Gem::DependencyInstaller.new(:document => [], :prerelease => prerelease)
 
+            # If we don't have a version, use the default version
+            version ||= Gem::Requirement.default
+
             begin
               installer.install(plugin_name, version)
             rescue Gem::GemNotFoundException