Allow nightly build to fail

Author: Disasm

riscv-rt/.travis.yml

@@ -13,6 +13,10 @@ env:
 
 if: (branch = staging OR branch = trying OR branch = master) OR (type = pull_request AND branch = master)
 
+matrix:
+  allow_failures:
+    - rust: nightly
+
 install:
   - ci/install.sh