Revert "Allow powerpc-unknown-linux-gnu build job to fail"

This reverts commit e88b825.

Author: mati865

.travis.yml

@@ -89,10 +89,6 @@ matrix:
         - shellcheck --version
         - shellcheck ci/*.sh
 
-  allow_failures:
-    # FIXME: https://github.com/rust-lang-nursery/stdsimd/issues/669
-    - name: "powerpc-unknown-linux-gnu - no assert_instr"
-
 install: travis_retry rustup target add $TARGET
 script:
   - cargo generate-lockfile