Add missing slash to installPath.
<?php

/*
 * TODO add file header and class description
 */

namespace Goatherd\Phpcs;

use Composer\Package\PackageInterface;

use Composer\Installer\LibraryInstaller;

class Installer extends LibraryInstaller
{
    /**
     * {@inheritDoc}
     */
    public function getInstallPath(PackageInterface $package)
    {
        // fix package name to denote single directory
        $name = str_replace(array('/', '\\'), '-', $package->getPrettyName());
        return $this->vendorDir . '/squizlabs/php_codesniffer/CodeSniffer/Standards/' . $name;
    }

    /**
     * {@inheritDoc}
     */
    public function supports($packageType)
    {
        return 'phpcs-standard' === $packageType;
    }
}

<?php

/*
 * TODO add file header and class description
 */

namespace Goatherd\Phpcs;

use Composer\Package\PackageInterface;

use Composer\Installer\LibraryInstaller;

class Installer extends LibraryInstaller
{
    /**
     * {@inheritDoc}
     */
    public function getInstallPath(PackageInterface $package)
    {
        // fix package name to denote single directory
        $name = str_replace(array('/', '\\'), '-', $package->getPrettyName());
        return $this->vendorDir . 'squizlabs/php_codesniffer/CodeSniffer/Standards/' . $name;
    }

    /**
     * {@inheritDoc}
     */
    public function supports($packageType)
    {
        return 'phpcs-standard' === $packageType;
    }
}
