Addresses strange behavior found investigating avajs#674

Since `babelConfig` in the CachingPrecompiler.prototype._factory method was a JavaScript object coming from an external source, the .push was altering the array in place. objectAssign is shallow so the plugins were being mutated, ensuring that AVA's required plugins were continually apending for each file loaded and parsed.

Using .concat ensures the original plugins array is not mutated and thus doesn't collect plugins like a hoarder.

lib/caching-precompiler.js

@@ -50,8 +50,7 @@ CachingPrecompiler.prototype._factory = function (babelConfig, cacheDir) {
 			ast: false
 		});
 
-		options.plugins = options.plugins || [];
-		options.plugins.push(powerAssert, transformRuntime, rewriteRuntime);
+		options.plugins = (options.plugins || []).concat([powerAssert, transformRuntime, rewriteRuntime]);
 
 		return options;
 	}