Remove composer.lock

[tanakahisateru]

I suggest to remove composer.lock and to ignore it because Composer doesn't use lock file when package installation. I prefer semantic version constraints instead.

TraviceCI can test sub package dependencies for both of latest version and lowest one. You can check if version constraint is appropriate or not.

language: php
php:
  - 5.6
  - 7
  - 7.1
  - 7.2
env:
  matrix:
    - DEPENDENCIES=""
    - DEPENDENCIES="--prefer-lowest"
before_script:
  - composer update $DEPENDENCIES
script:
  - ...

[Ippey]

@tanakahisateru Removed. Thanks!