Reduce dependency coupling for symfony/process

Move symfony/process to an optional dependency (required for dev, suggested otherwise).
This is a preparation for #78.

composer.json

@@ -17,16 +17,17 @@
         "predis/predis": "1.1.*",
         "monolog/monolog": "~1.7",
         "symfony/console": "~2.7|~3.0",
-        "symfony/yaml": "~2.7|~3.0",
-        "symfony/process": "~2.7|~3.0"
+        "symfony/yaml": "~2.7|~3.0"
     },
     "suggest": {
         "ext-proctitle": "Allows php-resque to rename the title of UNIX processes to show the status of a worker in PHP versions < 5.5.0.",
-        "ext-phpiredis": "Native PHP extension for Redis connectivity. Predis will automatically utilize when available."
+        "ext-phpiredis": "Native PHP extension for Redis connectivity. Predis will automatically utilize when available.",
+        "symfony/process": "To run the speed test command."
     },
     "require-dev": {
         "phpunit/phpunit": "~4.8",
-        "friendsofphp/php-cs-fixer": "~2.2.0"
+        "friendsofphp/php-cs-fixer": "~2.2.0",
+        "symfony/process": "~2.7|~3.0"
     },
     "bin": [
         "bin/resque"

src/Resque/Commands/SpeedTest.php

@@ -21,6 +21,8 @@
 use Symfony\Component\Process\Process;
 use Symfony\Component\Console\Helper\ProgressBar;
 
+use Exception;
+
 /**
  * Performs a raw speed test
  *
@@ -41,6 +43,10 @@ protected function configure()
 
     protected function execute(InputInterface $input, OutputInterface $output)
     {
+        if (!class_exists(Process::class)) {
+            throw new Exception('The Symfony process component is required to run the speed test.');
+        }
+
         Resque\Redis::setConfig(array('namespace' => 'resque:speedtest'));
 
         $testTime = (int)$input->getOption('time') ?: 5;