Fixed the symfony/config version constraint

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | N/A
| License       | MIT
| Doc PR        | N/A

##### This pull request fixes the `symfony/config` version constraint in the proxy manager bridge.

Before this pull, composer was only allowing `v2.3.0` to be downloaded. This surely was not the intention. I've updated the version constraint from `"2.3"` to `"~2.3"` thus allowing all 2.x versions that are equal to or greater than `v2.3.0`.