Prefer the locally installed version of AVA.

This hunts for AVA in the locally installed project, and runs that
CLI instead of the globally installed one.

Fixes avajs#157.

cli.js

@@ -1,6 +1,24 @@
 #!/usr/bin/env node
 'use strict';
 
+// Prefer the local installation of AVA.
+var resolveFrom = require('resolve-from');
+var localCLI;
+try {
+	localCLI = resolveFrom('.', 'ava/cli');
+} catch (e) {}
+
+if (localCLI && localCLI !== __filename) {
+	var thisVersion = require('./package.json').version;
+	var localVersion = require(localCLI.replace(/cli\.js$/, 'package.json')).version;
+	var warningMessage = 'Using local install of AVA (v%s), which ';
+	warningMessage += thisVersion === localVersion ? 'matches' : 'differs from';
+	warningMessage += ' global (v%s)';
+	console.warn(require('chalk').yellow(warningMessage), localVersion, thisVersion);
+	require(localCLI);
+	return;
+}
+
 var debug = require('debug')('ava');
 
 if (debug.enabled) {

readme.md

@@ -116,6 +116,9 @@ $ ava --help
 
 Files starting with `_` are ignored. This can be useful for having helpers in the same directory as your test files.
 
+*WARNING: NON-STANDARD BEHAVIOR:* The AVA CLI will always try to find and use your projects local install of AVA.
+ This is true even when you run the global `ava` command. This non-standard behavior solves an important [issue](https://github.com/sindresorhus/ava/issues/157),
+ and should have no impact on everyday use.
 
 ## Documentation