no explanation why the 2.7.11->2.8 bump:
https://github.com/mitchellh/vagrant/commit/025736c2823b1c4147c67af8c874496b37effa57

and listen 2.8 seems to have issues with sass:
https://github.com/sass/sass/pull/1527

see:
https://github.com/guard/listen/releases/tag/v2.8.1
--- vagrant-1.7.3/vagrant.gemspec~	2015-08-19 15:50:06.000000000 +0300
+++ vagrant-1.7.3/vagrant.gemspec	2015-08-19 15:50:29.068750880 +0300
@@ -19,7 +19,7 @@
   s.add_dependency "childprocess", ">= 0.6.0"
   s.add_dependency "erubis", "~> 2.7.0"
   s.add_dependency "i18n", ">= 0.6.0"
-  s.add_dependency "listen", "~> 3.1.5"
+  s.add_dependency "listen", "~> 2.7.11"
   s.add_dependency "hashicorp-checkpoint", "~> 0.1.5"
   s.add_dependency "log4r", "~> 1.1.9", "< 1.1.11"
   s.add_dependency "net-ssh", ">= 4.2.0"