Fix missing quotes

.travis.yml

@@ -103,7 +103,7 @@ anchors:
   clang-9:  &clang-9  { apt: { packages: [ "clang-9",
                                            "libc6-dbg",
                                            "libc++-dev",
-                                           "libstdc++-9-dev" ], sources: [ "llvm-toolchain-xenial-9,
+                                           "libstdc++-9-dev" ], sources: [ "llvm-toolchain-xenial-9",
                                                                            "ubuntu-toolchain-r-test"   ] } }                                                                         
     
   gcc-48:   &gcc-48   { apt: { packages: [ "g++-4.8"         ]                                           } }