Stop allowing Rust Stable Aarch64/ARM builds to fail on Travis CI.

It seems we've fixed all the root issues for these targets, so they
should be in as good of shape as other targets, at least with respect
to stability.

.travis.yml

@@ -573,12 +573,4 @@ matrix:
 
     # END GENERATED
 
-  allow_failures:
-    - env: TARGET_X=arm-unknown-linux-gnueabihf CC_X=arm-linux-gnueabihf-gcc CXX_X=arm-linux-gnueabihf-g++ MODE_X=DEBUG KCOV=0
-      rust: stable
-    - env: TARGET_X=aarch64-unknown-linux-gnu CC_X=aarch64-linux-gnu-gcc CXX_X=aarch64-linux-gnu-g++ MODE_X=DEBUG KCOV=0
-      rust: stable
-    - env: TARGET_X=aarch64-unknown-linux-gnu CC_X=aarch64-linux-gnu-gcc CXX_X=aarch64-linux-gnu-g++ MODE_X=RELWITHDEBINFO KCOV=0
-      rust: stable
-
 script: if [[ "$TARGET_X" =~ ^a*.*linux-.*eabi ]]; then travis_wait 60 mk/travis.sh; else mk/travis.sh; fi