Pin nightly rust version

fsck · fsck · commit 417a97ba67a7 · 2019-07-12T00:13:18.000+02:00
diff --git a/.travis.yml b/.travis.yml
@@ -1,6 +1,6 @@
 language: rust
 rust:
-  - nightly
+  - nightly-2019-07-09
 matrix:
   fast_finish: true
 cache: cargo
