Update version numbers for From impls

Author: ollie27

src/liballoc/string.rs

@@ -2008,7 +2008,7 @@ impl From<Box<str>> for String {
     }
 }
 
-#[stable(feature = "box_from_str", since = "1.18.0")]
+#[stable(feature = "box_from_str", since = "1.20.0")]
 impl From<String> for Box<str> {
     fn from(s: String) -> Box<str> {
         s.into_boxed_str()

src/libstd/ffi/c_str.rs

@@ -585,7 +585,7 @@ impl From<Box<CStr>> for CString {
     }
 }
 
-#[stable(feature = "box_from_c_string", since = "1.18.0")]
+#[stable(feature = "box_from_c_string", since = "1.20.0")]
 impl From<CString> for Box<CStr> {
     #[inline]
     fn from(s: CString) -> Box<CStr> {

src/libstd/ffi/os_str.rs

@@ -542,7 +542,7 @@ impl From<Box<OsStr>> for OsString {
     }
 }
 
-#[stable(feature = "box_from_os_string", since = "1.18.0")]
+#[stable(feature = "box_from_os_string", since = "1.20.0")]
 impl From<OsString> for Box<OsStr> {
     fn from(s: OsString) -> Box<OsStr> {
         s.into_boxed_os_str()

src/libstd/path.rs

@@ -1348,7 +1348,7 @@ impl From<Box<Path>> for PathBuf {
     }
 }
 
-#[stable(feature = "box_from_path_buf", since = "1.18.0")]
+#[stable(feature = "box_from_path_buf", since = "1.20.0")]
 impl From<PathBuf> for Box<Path> {
     fn from(p: PathBuf) -> Box<Path> {
         p.into_boxed_path()