minor #13624 removed composer --dev option everywhere (fabpot)

This PR was merged into the 2.3 branch.

Discussion
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removed composer --dev option everywhere

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

Commits
-------

0fe4913 renamed composer.phar to composer to be consistent with the Symfony docs
af59316 removed composer --dev option everywhere

Author: fabpot

README.md

@@ -95,5 +95,5 @@ Resources
 You can run the unit tests with the following command:
 
     $ cd path/to/Symfony/Component/HttpKernel/
-    $ composer.phar install
+    $ composer install
     $ phpunit