revert toolchain back to "test"

.travis.yml

@@ -1,5 +1,4 @@
 language: cpp
-cache: ccache
 matrix:
 # gcc
   include:
@@ -78,8 +77,8 @@ before_install:
   - if [ "$TRAVIS_OS_NAME" = "linux" ]; then
       eval "${MATRIX_EVAL}";
       if [ -n "$MATRIX_EVAL" ] && [ "$TRAVIS_COMPILER" != "clang" ]; then
-        sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 1E9377A2BA9EF27F;
-        sudo add-apt-repository -y ppa:ubuntu-toolchain-r/ppa;
+        sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 6B05F25D762E3157;
+        sudo add-apt-repository -y ppa:ubuntu-toolchain-r/test;
         sudo apt-get update -qq;
         sudo apt-get install -y $CC;
       fi;