Update mocha

- Update mocha.opts to use --require

When updating to Babel v7, this may need to be changed. For details,
see: https://github.com/mochajs/mocha/wiki/compilers-deprecation

packages/ripple-binary-codec/package.json

@@ -32,7 +32,7 @@
     "fs-extra": "^0.30.0",
     "intercept-stdout": "^0.1.2",
     "istanbul": "~0.4.3",
-    "mocha": "~2.4.5"
+    "mocha": "~6.2.0"
   },
   "scripts": {
     "compile": "babel --optional runtime -d distrib/npm/ src/ && cp src/enums/definitions.json distrib/npm/enums",

packages/ripple-binary-codec/test/mocha.opts

@@ -1 +1 @@
---reporter spec --slow 500 --compilers js:babel-register
+--reporter spec --slow 500 --require babel-core/register