drop node 6 support

We will no longer test on node6 and instead focus on 8+. This won't
break node6 immediately, but we will no longer go out of our way
supporting a version that does not receive security updates.

Signed-off-by: Claudius <opensource@amenthes.de>

.travis.yml

@@ -4,16 +4,10 @@ cache: yarn
 env:
   global:
     - CXX=g++-4.8
-    - YARN_VERSION=1.15.2
+    - YARN_VERSION=1.16.0
 
 jobs:
   include:
-    - env: task=npm-test
-      node_js:
-        - 6
-      before_install:
-        - curl -o- -L https://yarnpkg.com/install.sh | bash -s -- --version "$YARN_VERSION"
-        - export PATH="$HOME/.yarn/bin:$PATH"
     - env: task=npm-test
       node_js:
         - 8
@@ -26,6 +20,12 @@ jobs:
       before_install:
         - curl -o- -L https://yarnpkg.com/install.sh | bash -s -- --version "$YARN_VERSION"
         - export PATH="$HOME/.yarn/bin:$PATH"
+    - env: task=npm-test
+      node_js:
+        - 12
+      before_install:
+        - curl -o- -L https://yarnpkg.com/install.sh | bash -s -- --version "$YARN_VERSION"
+        - export PATH="$HOME/.yarn/bin:$PATH"
     - env: task=ShellCheck
       script:
         - shellcheck bin/heroku bin/setup