use "non-test" ppa

Maybe something weird about how gcc is configured with the test
toolchain?

.travis.yml

@@ -79,7 +79,7 @@ before_install:
       eval "${MATRIX_EVAL}";
       if [ -n "$MATRIX_EVAL" ] && [ "$TRAVIS_COMPILER" != "clang" ]; then
         sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 6B05F25D762E3157;
-        sudo add-apt-repository -y ppa:ubuntu-toolchain-r/test;
+        sudo add-apt-repository -y ppa:ubuntu-toolchain-r/ppa;
         sudo apt-get update -qq;
         sudo apt-get install -y $CC;
       fi;