Update unstable library feature list by Travis CI build 1590

Author: Travis CI

unstable_library_feature.txt

@@ -24,7 +24,7 @@ proc_macro_diagnostic               54140    2018-09-13 08:16:54 +0000  2018-09-
 proc_macro_quote                    54722    2018-10-01 10:42:16 -0700  2018-10-01 10:42:16 -0700
 proc_macro_def_site                 54724    2018-10-01 10:47:18 -0700  2018-10-01 10:47:18 -0700
 proc_macro_span                     54725    2018-10-01 10:53:03 -0700  2020-05-21 17:05:13 -0700
-is_sorted                           53485    2018-10-11 09:53:15 -0700  2018-10-11 09:53:15 -0700
+is_sorted                           53485    2018-10-11 15:59:54 -0700  2018-10-11 15:59:54 -0700
 thread_spawn_unchecked              55132    2018-10-16 22:42:14 +0200  2018-10-16 22:42:14 +0200
 hash_raw_entry                      56167    2018-11-22 09:52:24 -0700  2018-11-22 09:52:24 -0700
 exact_size_is_empty                 35428    2018-12-17 17:28:27 -0500  2018-12-17 17:28:27 -0500