build: change min node version to 8.3.0 (ipfs#996)

Also adjust Travis CI config to perform builds and tests with node 8.

`npm install`, `npm run build`, and `npm run test` all succeeded locally with
node version `8.3.0`.

The reason for version `8.3.0` instead of `8.0.0` is that `8.3.0` introduced
support for object rest/spread, which is used by at least one of this package's
devDependencies (`go-ipfs-dep`). It may be the case that at runtime older
versions of node would work correctly with the built package, but it seems
simpler to settle on the minimum supported version being a version that can
build, test, and use the package.

Closes ipfs#983.

.travis.yml

@@ -7,6 +7,7 @@ stages:
 
 node_js:
  - '10'
+ - '8'
 
 os:
  - linux

package.json

@@ -70,7 +70,7 @@
  "through2": "^3.0.1"
  },
  "engines": {
- "node": ">=10.0.0",
+ "node": ">=8.3.0",
  "npm": ">=3.0.0"
  },
  "repository": {