Update to use node version >= 8 and move 'ember-cli-htmlbars' under d…

…ependencies to resolve an error 'Addon templates were detected, but there are no template compilers registered for `ember-code-snippet`. Please make sure your template precompiler (commonly `ember-cli-htmlbars`) is listed in `dependencies` (NOT `devDependencies`) in `ember-code-snippet`'s `package.json`.'

.travis.yml

@@ -1,7 +1,7 @@
 ---
 language: node_js
 node_js:
-  - "6"
+  - "8"
 
 addons:
   chrome: stable

package.json

@@ -35,20 +35,20 @@
     "js-yaml": "^3.13.1",
     "rimraf": "^2.6.2",
     "semver": "^6.1.2",
-    "silent-error": "^1.1.1"
+    "silent-error": "^1.1.1",
+    "ember-cli-htmlbars": "^3.0.1"
   },
   "devDependencies": {
     "@ember/optional-features": "^0.7.0",
     "codeclimate-test-reporter": "^0.5.0",
-    "ember-cli": "~3.9.0",
+    "ember-cli": "^3.10.1",
     "ember-cli-addon-docs": "^0.6.13",
     "ember-cli-addon-docs-yuidoc": "^0.2.1",
     "ember-cli-dependency-checker": "^3.0.0",
     "ember-cli-deploy": "^1.0.2",
     "ember-cli-deploy-build": "^1.1.1",
     "ember-cli-deploy-git": "^1.3.3",
     "ember-cli-deploy-git-ci": "^1.0.1",
-    "ember-cli-htmlbars": "^3.0.1",
     "ember-cli-htmlbars-inline-precompile": "^2.0.0",
     "ember-cli-inject-live-reload": "^2.0.1",
     "ember-cli-shims": "^1.1.0",
@@ -74,7 +74,7 @@
     "testdouble": "^3.12.0"
   },
   "engines": {
-    "node": "^6.14.0 || ^8.10.0 || >= 10.*"
+    "node": "8.* || >= 10.*"
   },
   "ember-addon": {
     "configPath": "tests/dummy/config"