Explicitly enable ccache in travis build.

.travis.yml

@@ -115,11 +115,11 @@ before_install:
       sudo apt-get install gcc-8  $(if [ "$BACKEND" = cpp ]; then echo -n "g++-8"; fi )
       sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-8 60
       sudo update-alternatives --set gcc /usr/bin/gcc-8
-      export CC=gcc-8
+      export CC="ccache gcc-8"
       if [ "$BACKEND" = cpp ]; then
         sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-8 60
         sudo update-alternatives --set g++ /usr/bin/g++-8
-        export CXX=g++-8
+        export CXX="ccache g++-8"
       fi
       if [ -n "$CC" ]; then "$CC" --version; else gcc --version; fi
       if [ -n "$CXX" ]; then "$CXX" --version; else g++ --version; fi