Roll toolchain back to avoid a known ICE.

anp · anp · commit 3cd1acc1ad99 · 2019-06-20T00:27:12.000-07:00
Should be able to roll forward when rust-lang/rust#61872 is in a nightly.
diff --git a/.cargo/config b/.cargo/config
@@ -44,6 +44,6 @@ watch --clear
     -x fmt
     -x check-core
     -x test-core
-    -x clippy-core
     -x bench-core
 """
+#    -x clippy-core
diff --git a/rust-toolchain b/rust-toolchain
@@ -1 +1 @@
-nightly-2019-06-09
+nightly-2019-06-04
