Use native Node.js ESM loader, drop esm package

Node.js v13.2.0+ has ESM loader available unflagged.

Note the library can be used with Node.js v8+ using the esm package, but since
native ESM loader is only available in v13.2.0+, Travis tests only run on
v13/v14.

.travis.yml

@@ -1,9 +1,8 @@
 language: node_js
 
 node_js:
-- "lts/*"
 - "node"
-- "8"
+- "13"
 
 os:
 - linux

README.md

@@ -111,8 +111,9 @@ The library can be used in the browser by taking a local copy, or loading it fro
 
 ### Usage in Node.js
 
-The library can be used in a Node.js app from [npm](https://www.npmjs.com/package/geodesy) 
-(currently the [esm](https://www.npmjs.com/package/esm) package is required to load ES-modules):
+The library can be loaded from [npm](https://www.npmjs.com/package/geodesy) to be used in a Node.js app 
+(in Node.js v13.2.0+, or using the [esm](https://www.npmjs.com/package/esm) package in 
+v8.0.0–v12.15.0<sup title="v12.16.0+ is not compatible with esm@3.2.25">*</sup>):
 
 ```shell
 $ npm install geodesy esm

package.json

@@ -16,20 +16,19 @@
   },
   "bugs": "https://github.com/chrisveness/geodesy/issues",
   "scripts": {
-    "test": "mocha --exit -r esm test/*.js",
+    "test": "mocha --exit test/*.js",
     "lint": "eslint .",
     "cover": "c8 -r html npm test",
     "docs": "jsdoc *.js README.md -d ../../geodesy/docs"
   },
   "devDependencies": {
     "babel-eslint": "^10.0.0",
     "c8": "^7.0.0",
-    "chai": "^4.0.0",
+    "chai": "chaijs/chai#pull/1317/head",
     "coveralls": "^3.0.0",
     "eslint": "^6.0.0",
-    "esm": "^3.0.0",
     "jsdoc": "^3.0.0",
-    "mocha": "^6.0.0"
+    "mocha": "^7.0.0"
   },
   "eslintConfig": {
     "env": {