switch back to old nightly for the arm-gnueabi target

.travis.yml

@@ -9,6 +9,8 @@ matrix:
   include:
     - env: TARGET=aarch64-unknown-linux-gnu
     - env: TARGET=arm-unknown-linux-gnueabi
+      # FIXME(rust-lang/rust#36518)
+      rust: nightly-2016-09-21
     - env: TARGET=arm-unknown-linux-gnueabihf
     - env: TARGET=armv7-unknown-linux-gnueabihf
     - env: TARGET=i586-unknown-linux-gnu