Modifies Apache vhost definition to use single quotes

Per #395, usage of double-quotes attempts to expand the string
`${APACHE_LOG_DIR}` within the vhost definition. Since that value may or
may not be set, and, if set, may not be correct for the vagrant image,
single quotes must be used to prevent expansion.

Tested and verified.

Vagrantfile

@@ -9,7 +9,7 @@ apt-get update
 apt-get install -y apache2 git curl php7.0 php7.0-bcmath php7.0-bz2 php7.0-cli php7.0-curl php7.0-intl php7.0-json php7.0-mbstring php7.0-opcache php7.0-soap php7.0-sqlite3 php7.0-xml php7.0-xsl php7.0-zip libapache2-mod-php7.0
 
 # Configure Apache
-echo "<VirtualHost *:80>
+echo '<VirtualHost *:80>
 	DocumentRoot /var/www/public
 	AllowEncodedSlashes On
 
@@ -23,7 +23,7 @@ echo "<VirtualHost *:80>
 
 	ErrorLog ${APACHE_LOG_DIR}/error.log
 	CustomLog ${APACHE_LOG_DIR}/access.log combined
-</VirtualHost>" > /etc/apache2/sites-available/000-default.conf
+</VirtualHost>' > /etc/apache2/sites-available/000-default.conf
 a2enmod rewrite
 service apache2 restart