Update Mocha & Istanbul (#68)

We were on a really old version of Mocha, and standard practice for JS
code coverage is to use `nyc` now. This switches our code coverage tool
to `nyc`, and gets us on the latest version of Mocha.

Our code coverage did not appear to be working before either,
now it does.

Additionally, Mocha now supports `.mocharc.js` files which let us have
inline comments for configuration. `.mocha.opts` is deprecated, so I
have switched our Mocha configuration to the new format.

If we had configuration that was equal to the default, I have removed it

packages/ripple-keypairs/package.json

@@ -32,8 +32,8 @@
     "eslint-plugin-import": "^2.20.0",
     "eslint-plugin-mocha": "^6.2.2",
     "eslint-plugin-prettier": "^3.1.2",
-    "istanbul": "~0.3.5",
-    "mocha": "~2.3.3",
+    "mocha": "^7.0.0",
+    "nyc": "^15.0.0",
     "prettier": "^1.19.1",
     "typescript": "^3.1.3"
   },
@@ -41,7 +41,7 @@
     "compile": "tsc",
     "compile-babel": "babel --optional runtime -d distrib/npm/ src/",
     "compile-with-source-maps": "babel --optional runtime -s -t -d distrib/npm/ src/",
-    "test": "tsc && istanbul test _mocha",
+    "test": "tsc && nyc mocha",
     "lint": "eslint . --ext .ts --fix"
   },
   "repository": {