Improve the way submodules are updated [Close #153]

Also updated all plugins to their latest master versions.

README.md

@@ -56,7 +56,7 @@ Upgrading is easy.
 ```bash
 cd ~/.yadr
 git pull origin master
-rake install
+rake update
 ```
 
 # What's included, and how to customize?
@@ -455,10 +455,6 @@ YADR comes with a dead simple plugin manager that just uses git submodules, with
 
     yav -u https://github.com/airblade/vim-rooter
 
-You can update all the plugins easily:
-
-    yuv
-
 Delete a plugin (Coming Soon)
 
    ydv -p airblade-vim-rooter