update empower-core and babel-plugin-espower to make t._capt and t._expr unnecessary

- babel-plugin-espower 2.1.0 embeds value capturing helper (`_capt` and `_expr`) into transpiled code. Therefore, copying `t._capt` and `t._expr` is not required any more
- In addition, babel-plugin-espower 2.1.0 is refactored a lot to reduce dependencies
- empower-core 0.4.0 stop adding `_capt` and `_expr` methods into assertion object

refs: #302 (comment)
refs: power-assert-js/babel-plugin-espower#11
refs: twada/empower-core#3

Author: twada

lib/test.js

@@ -284,8 +284,5 @@ Test.prototype._publicApi = function () {
 		api[el] = enhanced[el].bind(enhanced);
 	});
 
-	api._capt = enhanced._capt.bind(enhanced);
-	api._expr = enhanced._expr.bind(enhanced);
-
 	return api;
 };

package.json

@@ -79,7 +79,7 @@
     "arrify": "^1.0.0",
     "ava-init": "^0.1.0",
     "babel-core": "^6.3.21",
-    "babel-plugin-espower": "^2.0.0",
+    "babel-plugin-espower": "^2.1.0",
     "babel-plugin-transform-runtime": "^6.3.13",
     "babel-preset-es2015": "^6.3.13",
     "babel-preset-stage-2": "^6.3.13",
@@ -93,7 +93,7 @@
     "core-assert": "^0.1.0",
     "debug": "^2.2.0",
     "deeper": "^2.1.0",
-    "empower-core": "^0.2.0",
+    "empower-core": "^0.4.0",
     "figures": "^1.4.0",
     "find-cache-dir": "^0.1.1",
     "fn-name": "^2.0.0",