Updated Node environment.

.travis.yml

@@ -2,7 +2,7 @@
 
 language: node_js
 node_js:
-  - "10"
+  - "node"
 cache: yarn
 
 script:

package.json

@@ -7,17 +7,17 @@
   "dependencies": {
     "bootstrap-sass": "^3.3.7",
     "browser-sync": "^2.18.13",
-    "cssnano": "^3.10.0",
-    "gulp": "^3.9.1",
+    "cssnano": "^4.1.7",
+    "gulp": "^4.0.0",
     "gulp-changed": "^3.1.0",
-    "gulp-clean-css": "^3.7.0",
+    "gulp-clean-css": "^4.0.0",
     "gulp-concat": "^2.6.1",
-    "gulp-htmlmin": "^3.0.0",
-    "gulp-image-resize": "^0.12.0",
+    "gulp-htmlmin": "^5.0.1",
+    "gulp-image-resize": "^0.13.0",
     "gulp-md5-plus": "^1.0.3",
-    "gulp-minify": "^1.0.0",
-    "gulp-postcss": "^7.0.0",
-    "gulp-sass": "^3.1.0",
+    "gulp-minify": "^3.1.0",
+    "gulp-postcss": "^8.0.0",
+    "gulp-sass": "^4.0.2",
     "gulp-util": "^3.0.8",
     "merge-stream": "^1.0.1",
     "run-sequence": "^2.1.0",