Adds composer.json dependency on guzzlehttp/guzzle for acquia#54

This is the same version of Guzzle used in [Drupal 8.0.0-beta10](http://cgit.drupalcode.org/drupal/tree/core/composer.json?id=8.0.0-beta10#n23).

    $ composer require "guzzlehttp/guzzle:~5.0"
    ./composer.json has been updated
    Loading composer repositories with package information
    Initializing PEAR repository http://pear.php.net
    Updating dependencies (including require-dev)
      - Installing react/promise (v2.2.0)
        Loading from cache

      - Installing guzzlehttp/streams (3.0.0)
        Loading from cache

      - Installing guzzlehttp/ringphp (1.0.7)
        Loading from cache

      - Installing guzzlehttp/guzzle (5.2.0)
        Loading from cache

    Writing lock file
    Generating autoload files

composer.json

@@ -14,6 +14,7 @@
     "require": {
         "php": ">=5.3.0",
         "cpliakas/psolr": ">=0.4",
+        "guzzlehttp/guzzle": "~5.0",
         "guzzle/service": "~3.0",
         "lstrojny/fxmlrpc": ">=0.8,<0.9",
         "pear/net_dns2": "1.3.2",