chore: remove testing on Node6 CircleCI

Node 6 LTS finish on April 2019
https://github.com/nodejs/Release#nodejs-release-working-group
This is just the begining to remove Node6 support, total support will take part on April 2019

.circleci/config.yml

@@ -3,9 +3,6 @@ version: 2
 aliases:
   - &defaults
     working_directory: ~/verdaccio
-  - &node6_executor
-    docker:
-      - image: circleci/node:6
   - &node8_executor
     docker:
       - image: circleci/node:8
@@ -94,15 +91,6 @@ jobs:
           paths:
             - ./*
 
-  test_node6:
-    <<: *defaults
-    <<: *node6_executor
-    steps:
-      - *restore_repo
-      - run:
-          name: Test with Node 6
-          command: yarn run test
-
   test_node8:
     <<: *defaults
     <<: *node8_executor
@@ -207,10 +195,6 @@ workflows:
     jobs:
       - prepare:
           <<: *ignore_non_dev_branches
-      - test_node6:
-          requires:
-            - prepare
-          <<: *ignore_non_dev_branches
       - test_node8:
           requires:
             - prepare
@@ -237,7 +221,6 @@ workflows:
           <<: *ignore_non_dev_branches
       - coverage:
           requires:
-            - test_node6
             - test_node8
             - test_node9
             - test_node10