travis.yml: remove older node versions from test

mocha has a dependency on yargs which only supports actively
maintained Node.js releases, which means they outlaw 6 and 8.  At the
moment they're OK with 10, but only because they haven't released
since it reached EOL on 2021-04-30.

Drop the versions that won't build in travis as of today.  We'll have
to test manually to confirm old Node.js release pass.

Signed-off-by: Peter A. Bigot <pab@pabigot.com>

.travis.yml

@@ -4,8 +4,7 @@ node_js:
   - "16"
   - "14"
   - "12"
-  - "8"
-  - "6"
+  - "10"
 branches:
   only:
     - master