Change assertion library to chai

package.json

@@ -29,7 +29,9 @@
     "spectron": "^3.7.2"
   },
   "devDependencies": {
+    "@types/chai": "^4.1.2",
     "@types/mocha": "^2.2.47",
+    "chai": "^4.1.2",
     "dts-bundle": "^0.7.3",
     "electron": "^1.7.11",
     "electron-prebuilt": "^1.4.13",

yarn.lock

@@ -2,6 +2,10 @@
 # yarn lockfile v1
 
 
+"@types/chai@^4.1.2":
+  version "4.1.2"
+  resolved "https://registry.yarnpkg.com/@types/chai/-/chai-4.1.2.tgz#f1af664769cfb50af805431c407425ed619daa21"
+
 "@types/[email protected]":
   version "0.1.30"
   resolved "https://registry.yarnpkg.com/@types/detect-indent/-/detect-indent-0.1.30.tgz#dc682bb412b4e65ba098e70edad73b4833fb910d"
@@ -518,6 +522,17 @@ center-align@^0.1.1:
     align-text "^0.1.3"
     lazy-cache "^1.0.3"
 
+chai@^4.1.2:
+  version "4.1.2"
+  resolved "https://registry.yarnpkg.com/chai/-/chai-4.1.2.tgz#0f64584ba642f0f2ace2806279f4f06ca23ad73c"
+  dependencies:
+    assertion-error "^1.0.1"
+    check-error "^1.0.1"
+    deep-eql "^3.0.0"
+    get-func-name "^2.0.0"
+    pathval "^1.0.0"
+    type-detect "^4.0.0"
+
 chalk@^1.1.3:
   version "1.1.3"
   resolved "https://registry.yarnpkg.com/chalk/-/chalk-1.1.3.tgz#a8115c55e4a702fe4d150abd3872822a7e09fc98"