try! -> ?

Automated conversion using the untry tool [1] and the following command:

```
$ find -name '*.rs' -type f | xargs untry
```

at the root of the Rust repo.

[1]: https://github.com/japaric/untry

1 files changed, 2 insertions, 2 deletions

diff --git a/src/libstd/ffi/c_str.rs b/src/libstd/ffi/c_str.rs
index 1db45764552..52d7bb128d5 100644
--- a/src/libstd/ffi/c_str.rs
+++ b/src/libstd/ffi/c_str.rs
@@ -318,9 +318,9 @@ impl From<CString> for Vec<u8> {
 #[stable(feature = "cstr_debug", since = "1.3.0")]
 impl fmt::Debug for CStr {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        try!(write!(f, "\""));
+        write!(f, "\"")?;
         for byte in self.to_bytes().iter().flat_map(|&b| ascii::escape_default(b)) {
-            try!(f.write_char(byte as char));
+            f.write_char(byte as char)?;
         }
         write!(f, "\"")
     }