fix(jest): Update deprecated ts-jest config option.

This is consistently reproducible -- see e.g.,
[the latest Travis build](https://travis-ci.org/microsoft/TypeScript-Node-Starter/builds/533095625)
for an example of this error in CI. Simple one-line fix.

jest.config.js

@@ -1,7 +1,7 @@
 module.exports = {
 	globals: {
 		'ts-jest': {
-			tsConfigFile: 'tsconfig.json'
+			tsConfig: 'tsconfig.json'
 		}
 	},
 	moduleFileExtensions: [
@@ -15,4 +15,4 @@ module.exports = {
 		'**/test/**/*.test.(ts|js)'
 	],
 	testEnvironment: 'node'
-};
+};