Ignore engines when installing on travis CI (which runs node 10) (#668)

Fixes issue with webpack 2.7.0 -> upath 1.0.5 dependency, which incorrectly
specifies compatibility with node <= 9

Issue described here: anodynos/upath#15

.travis.yml

@@ -1,6 +1,7 @@
 language: node_js
 node_js:
   - "node"
+install: "yarn install --ignore-engines"
 before_script: "npm install -g codeclimate-test-reporter"
 script:
   - yarn run lint