Allow failures for powerpc and powerpc64

The two platforms have less hardware support than powerpc64le and
currently have some rustc-related issues that prevent investigating
CI failures properly.

Author: lu-zero

.travis.yml

@@ -56,6 +56,8 @@ matrix:
   allow_failures:
     - env: CLIPPY=On TARGET=x86_64-unknown-linux-gnu NO_ADD=1
     - env: RUSTFMT=On TARGET=x86_64-unknown-linux-gnu NO_ADD=1
+    - env: TARGET=powerpc-unknown-linux-gnu
+    - env: TARGET=powerpc64-unknown-linux-gnu
 
 before_install:
   # FIXME (travis-ci/travis-ci#8920) shouldn't be necessary...