[Slim] Add PHPLint tool to check syntax

Created by: ybelenko

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: master, 3.3.x, 4.0.x. Default: master.
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Description of the PR

PHPLint is a tool that can speed up linting of php files by running several lint processes at once.

Ref: overtrue/phplint

$ composer run phplint
> phplint ./ --exclude=vendor
phplint 1.0.2 by overtrue and contributors.

No config file loaded.

.......................................     40 / 87 (45%)
........................................     80 / 87 (91%)
........

Time: 3 secs    Memory: 2.0 MiB Cache: No

OK! (Files: 87, Success: 87)

Upgrade note

We have to increase required PHP version to 5.5.9, because it's minimum version for PHPLint tool.

cc @jebentier @dkarlovi @mandrean @jfastnacht @ackintosh