minor #778 Move symfony/process dependency from require-dev to require (franmomu)

This PR was merged into the 1.0-dev branch.

Discussion
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Move symfony/process dependency from require-dev to require

Hi!

I was trying to use [`MakerTestCase`](https://github.com/symfony/maker-bundle/blob/a47408fa6e39034a5abe324a46f4e6733266267a/src/Test/MakerTestCase.php) and I'm finding some little things.

In this case, `symfony/process` is being used under the src directory in two files:
- [src/Test/MakerTestProcess.php](https://github.com/symfony/maker-bundle/blob/a47408fa6e39034a5abe324a46f4e6733266267a/src/Test/MakerTestProcess.php#L14)
- [src/Test/MakerTestEnvironment.php](https://github.com/symfony/maker-bundle/blob/a47408fa6e39034a5abe324a46f4e6733266267a/src/Test/MakerTestEnvironment.php#L18-L19)

So trying to execute tests I'm getting: `Error: Class 'Symfony\Component\Process\Process' not found`

Commits
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156134f Throw an exception if symfony/process is not installed

Author: weaverryan

src/Test/MakerTestCase.php

@@ -15,6 +15,7 @@
 use PHPUnit\Framework\TestCase;
 use Symfony\Bundle\MakerBundle\MakerInterface;
 use Symfony\Bundle\MakerBundle\Str;
+use Symfony\Component\Process\Process;
 
 abstract class MakerTestCase extends TestCase
 {
@@ -32,6 +33,10 @@ abstract public function getTestDetails();
 
     protected function executeMakerCommand(MakerTestDetails $testDetails)
     {
+        if (!class_exists(Process::class)) {
+            throw new \LogicException('The MakerTestCase cannot be run as the Process component is not installed. Try running "compose require --dev symfony/process".');
+        }
+
         if (!$testDetails->isSupportedByCurrentPhpVersion()) {
             $this->markTestSkipped();
         }