Updated composer.json to allow newer versions of ZF2.

I've had to resort to forking phpDocumentor and updating composer.json in order to use it in my projects requiring Zend Framework 2.3.x.

After talking a bit with @rdohms and seeing his comment [here](https://github.com/phpDocumentor/phpDocumentor2/issues/1385#issuecomment-59050707), I decided to prepare this PR along with tests in hopes of being able to remove my fork.

As a base case, let's take a developer who is using PHP 5.3.3 and wanting the current 2.7.0 release of phpDocumentor.

```
vagrant@packer-vmware-iso:~/my-app$ php -v
PHP 5.3.3 (cli) (built: Oct 15 2014 16:54:14)
Copyright (c) 1997-2010 The PHP Group
Zend Engine v2.3.0, Copyright (c) 1998-2010 Zend Technologies
vagrant@packer-vmware-iso:~/my-app$ cat composer.json
{
    "name": "vagrant/my-app",
    "repositories": [
        {
            "url": "../phpDocumentor2",
            "type": "git"
        }
    ],
    "require-dev": {
        "phpdocumentor/phpdocumentor": "dev-master"
    }
}
```

`../phpDocumentor2` is a local clone of the upstream repo, currently referencing commit 98b28ee43b314063e759882d3dcece16afc4d9b2 (i.e. 2.7.0).

```
vagrant@packer-vmware-iso:~/my-app$ composer install --dev
Loading composer repositories with package information
Installing dependencies (including require-dev)
  - Installing zendframework/zend-stdlib (2.1.6)
  --snip--
Writing lock file
Generating autoload files
```

I omitted most of the output for brevity, but as you can see, it installs 2.1.x ZF2 packages (as expected).

Now, what if the PR is applied?

```
diff --git a/composer.json b/composer.json
index 7e89d06..de825c3 100644
--- a/composer.json
+++ b/composer.json
@@ -30,13 +30,13 @@
         "jms/serializer":           "~0.12",
         "knplabs/knp-menu":         "~1.1",

-        "zendframework/zend-stdlib":         "2.1.*",
-        "zendframework/zend-servicemanager": "2.1.*",
-        "zendframework/zend-config":         "2.1.*",
-        "zendframework/zend-i18n":           "2.1.*",
-        "zendframework/zend-serializer":     "2.1.*",
-        "zendframework/zend-cache":          "2.1.*",
-        "zendframework/zend-filter":         "2.1.*",
+        "zendframework/zend-stdlib":         "~2.1",
+        "zendframework/zend-servicemanager": "~2.1",
+        "zendframework/zend-config":         "~2.1",
+        "zendframework/zend-i18n":           "~2.1",
+        "zendframework/zend-serializer":     "~2.1",
+        "zendframework/zend-cache":          "~2.1",
+        "zendframework/zend-filter":         "~2.1",

         "phpdocumentor/graphviz":            "~1.0",
         "phpdocumentor/fileset":             "~1.0",
diff --git a/composer.lock b/composer.lock
index de4c2c7..246081e 100644
--- a/composer.lock
+++ b/composer.lock
-- snip ---
```

`composer.lock` diff omitted for brevity.

```
vagrant@packer-vmware-iso:~/my-app$ rm -rf vendor/* && rm composer.lock
vagrant@packer-vmware-iso:~/my-app$ composer install --dev
Loading composer repositories with package information
Installing dependencies (including require-dev)
  - Installing zendframework/zend-stdlib (2.2.6)
  --snip--
Writing lock file
Generating autoload files
```

This time, we get 2.2.6. (yes, ZF2 went up to 2.2.8, but the individual components required by phpDocumentor stopped at 2.2.6).

Now, what if our application requires at least ZF2 version 2.3? Obviously, this won't work on our PHP 5.3.3 VM, but if we upgrade to 5.5.x...

```
vagrant@packer-vmware-iso:~/my-app$ php -v
PHP 5.5.9-1ubuntu4.4 (cli) (built: Sep  4 2014 06:56:34)
Copyright (c) 1997-2014 The PHP Group
Zend Engine v2.5.0, Copyright (c) 1998-2014 Zend Technologies
    with Zend OPcache v7.0.3, Copyright (c) 1999-2014, by Zend Technologies
vagrant@packer-vmware-iso:~/my-app$ rm -rf vendor/* && rm composer.lock
vagrant@packer-vmware-iso:~/my-app$ composer require "zendframework/zendframework":"~2.3"
./composer.json has been updated
Loading composer repositories with package information
Installing dependencies (including require-dev)
  - Installing zendframework/zendframework (2.3.3)
  --snip--
Writing lock file
Generating autoload files
```

I hope this testing is sufficient to ease any concerns, and that you will merge this PR.