Use COMPOSER_ROOT_VERSION to support circular dependency to root

We currently have an indirect cyclic dependency between
socket -> socket-client -> dns -> socket
which causes the test setup to fail otherwise.

.travis.yml

@@ -11,7 +11,7 @@ php:
 sudo: false
 
 install:
-  - composer install --prefer-source --no-interaction
+  - COMPOSER_ROOT_VERSION=`git describe --abbrev=0` composer install --no-interaction
 
 script:
   - phpunit --coverage-text

README.md

@@ -16,6 +16,7 @@ and [`Stream`](https://github.com/reactphp/stream) components.
   * [ConnectionInterface](#connectioninterface)
     * [getRemoteAddress()](#getremoteaddress)
 * [Install](#install)
+* [Tests](#tests)
 * [License](#license)
 
 ## Quickstart example
@@ -144,6 +145,23 @@ $ composer require react/socket:^0.4.4
 
 More details about version upgrades can be found in the [CHANGELOG](CHANGELOG.md).
 
+## Tests
+
+To run the test suite, you first need to clone this repo and then install all
+dependencies [through Composer](http://getcomposer.org).
+Because the test suite contains some circular dependencies, you may have to
+manually specify the root package version like this:
+
+```bash
+$ COMPOSER_ROOT_VERSION=`git describe --abbrev=0` composer install
+```
+
+To run the test suite, you need PHPUnit. Go to the project root and run:
+
+```bash
+$ phpunit
+```
+
 ## License
 
 MIT, see [LICENSE file](LICENSE).