Update to latest Node v6.x release

Package installation fails with Node v6.2 due to an incompatible
"check-dependencies" package:
https://travis-ci.org/hypothesis/client/jobs/228937886

Author: robertknight

.travis.yml

@@ -9,9 +9,9 @@ matrix:
     # 'node_js' is listed for each job to ensure that only two builds are run.
     # See https://github.com/hypothesis/client/pull/27#discussion_r70611726
     - env: ACTION=lint
-      node_js: '6.2'
+      node_js: '6.10'
       script: yarn run lint
     - env: ACTION=test
-      node_js: '6.2'
+      node_js: '6.10'
       after_success:
         yarn run report-coverage