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問題点
.htaccessが許可されていない。

解決法
"AllowOverride All"を000-default.conf.confにいれて解決
<Directory "/var/www/html">
   AllowOverride All
</Directory>
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問題点
RewriteEngineが許可されていない。

[Wed May 17 02:29:31.273801 2017] [core:alert] [pid 1755] [client 192.168.131.10:38705] /var/www/html/.htaccess: Invalid command 'RewriteEngine', perhaps misspelled or defined by a module not included in the server configuration

解決法
sudo a2enmod rewrite
service apache2 restart