Update pom.xml to include slf4j

[dpastov]

<dependencies>
	<dependency>
		<groupId>org.opensaml</groupId>
		<artifactId>opensaml</artifactId>
		<version>2.6.4</version>
	</dependency>

	<dependency>
		<groupId>javax.xml.bind</groupId>
		<artifactId>jaxb-api</artifactId>
		<version>2.3.1</version>
	</dependency>

	<dependency>
		<groupId>org.slf4j</groupId>
		<artifactId>slf4j-api</artifactId>
		<version>1.7.36</version>
	</dependency>

	<dependency>
		<groupId>org.slf4j</groupId>
		<artifactId>slf4j-simple</artifactId>
		<version>1.7.36</version>
	</dependency>
</dependencies>