See that ccache is actually being used by travis.

.travis.yml

@@ -118,8 +118,8 @@ before_install:
       for i in {1..10}; do sudo apt-get update && sudo apt-get install --yes gcc-8  $(if [ "$BACKEND" = cpp ]; then echo -n "g++-8"; fi ) && break; sleep 2; done
       sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-8 60 $(if [ "$BACKEND" = cpp ]; then echo " --slave /usr/bin/g++ g++ /usr/bin/g++-8"; fi)
       sudo update-alternatives --set gcc /usr/bin/gcc-8
-      export CC=gcc-8
-      if [ "$BACKEND" = cpp ]; then sudo update-alternatives --set g++ /usr/bin/g++-8; export CXX=g++-8; fi
+      export CC=gcc
+      if [ "$BACKEND" = cpp ]; then sudo update-alternatives --set g++ /usr/bin/g++-8; export CXX=g++; fi
     fi
 
   - |
@@ -134,8 +134,8 @@ before_install:
       fi
     fi
 
-  - if [ -n "$CC" ]; then $CC --version; fi
-  - if [ -n "$CXX" ]; then $CXX --version; fi
+  - if [ -n "$CC" ]; then which $CC; $CC --version; fi
+  - if [ -n "$CXX" ]; then which $CXX; $CXX --version; fi
 
   - if [[ "$STACKLESS" == "true" ]]; then
       conda config --add channels stackless;