Use COMPOSER_ROOT_VERSION to support circular dependency to root

We currently have a cyclic dependency between react/socket and react/dns
which causes the test setup to fail or pick outdated versions otherwise.

Author: clue

.travis.yml

@@ -21,7 +21,7 @@ matrix:
 sudo: false
 
 install:
-  - composer install --no-interaction
-  
+  - COMPOSER_ROOT_VERSION=`git describe --abbrev=0` composer install --no-interaction
+
 script:
   - vendor/bin/phpunit --coverage-text

README.md

@@ -108,10 +108,12 @@ 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):
+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 install
+$ COMPOSER_ROOT_VERSION=`git describe --abbrev=0` composer install
 ```
 
 To run the test suite, go to the project root and run: