servo: Merge #8446 - Rust upgrade to rustc 1.6.0-nightly (5b4986fa5 2015-11-08) (from servo:rustup_20151110); r=SimonSapin+Ms2ger+jdm+Manishearth

<s>DO NOT r+ or try+ this</s>

<s>It causes an OOM (https://github.com/rust-lang/rust/issues/29740) and can crash the OS. Probably will set our CI on fire. </s>

Source-Repo: https://github.com/servo/servo
Source-Revision: f13c72d68e1e7f49e241938bfd6e8a588c68e86b

servo/python/tidy.py

@@ -164,7 +164,7 @@ def check_lock(file_name, contents):
     packages = {}
 
     # package names to be neglected (as named by cargo)
-    exceptions = []
+    exceptions = ["libc"]
 
     while idx < len(contents):
         content = contents[idx].strip()