Removing "target-dir" option to stop extra nesting

There is no valid reason to set this since it causes extra nesting:
    ./vendor/twitter/bootstrap/twitter/bootstrap/

composer.json

@@ -5,6 +5,5 @@
   , "homepage": "http://twitter.github.com/bootstrap/"
   , "author": "Twitter Inc."
   , "license": "Apache-2.0"
-  , "target-dir": "twitter/bootstrap"
 
 }