composer installしたら"continue" targeting switch is equivalent to "break". Did you mean to use "continue 2"? となったときの対応方法

- 環境
    - macOS Mojave バージョン10.14.2

# 事象 : composer installしたら怒られた
```json:composer.json
{
    "name": "tryphp/qiita",
    "description": "Sample For Qiita post.",
    "type": "project",
    "require": {
        "php": "7.3.1"
    },
    "require-dev": {
        "phpunit/phpunit": "7.5.2"
    },
    "license": "MIT",
    "minimum-stability": "dev"
}
```
```bash:事象
$ composer install
Loading composer repositories with package information
Updating dependencies (including require-dev)

                                                                                           
  [ErrorException]                                                                         
  "continue" targeting switch is equivalent to "break". Did you mean to use "continue 2"?  
                                                                                           

install [--prefer-source] [--prefer-dist] [--dry-run] [--dev] [--no-dev] [--no-custom-installers] [--no-autoloader] [--no-scripts] [--no-progress] [-v|vv|vvv|--verbose] [-o|--optimize-autoloader] [-a|--classmap-authoritative] [--ignore-platform-reqs] [--] [<packages>]...
```

## 原因 : composer自体が古い
- [&quot;continue&quot; targeting switch is equivalent to &quot;break&quot;. Did you mean to use &quot;continue 2&quot;? · Issue #945 · botman/botman](https://github.com/botman/botman/issues/945)

## 対応 : composer自体をアップデートする
```bash:対応
# composer自体をアップデートする
$ composer self-update
Updating to version 1.8.0 (stable channel).
    Downloading: 100%         
Use composer self-update --rollback to return to version 1.1.3

# バージョン確認
$ composer --version
Composer version 1.8.0 2018-12-03 10:31:16

# もう一回composer installしてみる
$ composer install
Loading composer repositories with package information
Updating dependencies (including require-dev)
Package operations: 28 installs, 0 updates, 0 removals
  - Installing sebastian/version (2.0.1): Downloading (100%)         
  - Installing sebastian/resource-operations (dev-master 4d7a795): Cloning 4d7a795d35 from cache
  - Installing sebastian/object-reflector (dev-master 7707193): Cloning 7707193304 from cache
  - Installing sebastian/recursion-context (dev-master dbe1869): Cloning dbe1869c13 from cache
  - Installing sebastian/object-enumerator (dev-master e6be0a5): Cloning e6be0a5481 from cache
  - Installing sebastian/global-state (2.0.0): Downloading (100%)         
  - Installing sebastian/exporter (dev-master c8c4f19): Cloning c8c4f196e3 from cache
  - Installing sebastian/environment (dev-master 585fbcd): Cloning 585fbcdeff from cache
  - Installing sebastian/diff (dev-master 6906fbe): Cloning 6906fbe334 from cache
  - Installing sebastian/comparator (dev-master 33825cd): Cloning 33825cd25f from cache
  - Installing phpunit/php-timer (dev-master 9ef9968): Cloning 9ef9968ba2 from cache
  - Installing phpunit/php-text-template (1.2.1): Downloading (100%)         
  - Installing phpunit/php-file-iterator (dev-master 2da97ea): Cloning 2da97ea973 from cache
  - Installing theseer/tokenizer (1.1.0): Downloading (100%)         
  - Installing sebastian/code-unit-reverse-lookup (dev-master 22f5f5f): Cloning 22f5f5ff89 from cache
  - Installing phpunit/php-token-stream (dev-master 6df086c): Cloning 6df086c042 from cache
  - Installing phpunit/php-code-coverage (dev-master afe4b8a): Cloning afe4b8aa75 from cache
  - Installing doctrine/instantiator (dev-master 8c8c1af): Cloning 8c8c1afee0 from cache
  - Installing symfony/polyfill-ctype (v1.10.0): Downloading (100%)         
  - Installing webmozart/assert (1.4.0): Downloading (100%)         
  - Installing phpdocumentor/reflection-common (1.0.1): Downloading (100%)         
  - Installing phpdocumentor/type-resolver (0.4.0): Downloading (100%)         
  - Installing phpdocumentor/reflection-docblock (4.3.0): Downloading (100%)         
  - Installing phpspec/prophecy (dev-master 7e27218): Cloning 7e27218052 from cache
  - Installing phar-io/version (2.0.1): Downloading (100%)         
  - Installing phar-io/manifest (dev-master 7761fca): Cloning 7761fcacf0 from cache
  - Installing myclabs/deep-copy (1.x-dev 3e01bda): Cloning 3e01bdad3e from cache
  - Installing phpunit/phpunit (7.5.2): Downloading (100%)         
sebastian/global-state suggests installing ext-uopz (*)
phpunit/php-code-coverage suggests installing ext-xdebug (^2.6.1)
phpunit/phpunit suggests installing phpunit/php-invoker (^2.0)
phpunit/phpunit suggests installing ext-xdebug (*)
Writing lock file
Generating autoload files
```