{
    "name": "cakephp/app",
    "description": "CakePHP skeleton app",
    "homepage": "http://cakephp.org",
    "type": "project",
    "license": "MIT",
    "require": {
        "php": ">=5.4.16",
        "cakephp/cakephp": "~3.1",
        "mobiledetect/mobiledetectlib": "2.*",
        "cakephp/migrations": "~1.0",
        "cakephp/plugin-installer": "*"
    },
    "require-dev": {
        "psy/psysh": "@stable",
        "cakephp/debug_kit": "~3.2",
        "cakephp/bake": "~1.1"
    },
    "suggest": {
        "phpunit/phpunit": "Allows automated tests to be run without system-wide install.",
        "cakephp/cakephp-codesniffer": "Allows to check the code against the coding standards used in CakePHP."
    },
    "autoload": {
        "psr-4": {
            "App\\": "src"
        }
    },
    "autoload-dev": {
        "psr-4": {
            "App\\Test\\": "tests",
            "Cake\\Test\\": "./vendor/cakephp/cakephp/tests"
        }
    },
    "scripts": {
        "post-install-cmd": "App\\Console\\Installer::postInstall",
        "post-autoload-dump": "Cake\\Composer\\Installer\\PluginInstaller::postAutoloadDump"
    },
    "minimum-stability": "dev",
    "prefer-stable": true
}

{
    "name": "cakephp/app",
    "description": "CakePHP skeleton app",
    "homepage": "http://cakephp.org",
    "type": "project",
    "license": "MIT",
    "require": {
        "php": ">=5.4.16",
        "cakephp/cakephp": "~3.1",
        "mobiledetect/mobiledetectlib": "2.*",
        "cakephp/migrations": "~1.0",
        "cakephp/plugin-installer": "*"
    },
    "require-dev": {
        "psy/psysh": "@stable",
        "cakephp/debug_kit": "~3.2",
        "cakephp/bake": "~1.1"
    },
    "suggest": {
        "phpunit/phpunit": "Allows automated tests to be run without system-wide install.",
        "cakephp/cakephp-codesniffer": "Allows to check the code against the coding standards used in CakePHP."
    },
    "autoload": {
        "psr-4": {
            "App\\": "src"
        }
    },
    "autoload-dev": {
        "psr-4": {
            "App\\Test\\": "tests",
            "Cake\\Test\\": "./vendor/cakephp/cakephp/tests"
        }
    },
    "scripts": {
        "post-install-cmd": "App\\Console\\Installer::postInstall",
        "post-autoload-dump": "Cake\\Composer\\Installer\\PluginInstaller::postAutoloadDump"
    },
    "minimum-stability": "stable",
    "prefer-stable": true
}

Change min. stability to "stable".
Change min. stability to "stable".

With the stable release of cakephp 3.1 and related projects it's better to use stable packages.