Update spec-runner to atom-mocha@2.0.3

APM appears to have stopped packaging a module's entry-point implicitly,
unlike NPM (which always includes the file specified by the `main` field
in package.json). The latest version now lists `index.js` in the `files`
array, allowing package specs to run after `apm install`.

/cc @hansonw

package.json

@@ -19,7 +19,7 @@
 		"print": "*"
 	},
 	"devDependencies": {
-		"atom-mocha": ">=2.0.1",
+		"atom-mocha": ">=2.0.3",
 		"get-options": "*",
 		"coffee-script": "*",
 		"rimraf": "*",