Update pom.xml to ensure compatible Java compiler

When compiling your project as-is I encountered the error "annotations are not supported in -source 1.3 (use -source 5 or higher to enable annotations)".

I added 
```
		<pluginManagement>
			<plugins>
				<plugin>
					<groupId>org.apache.maven.plugins</groupId>
					<artifactId>maven-compiler-plugin</artifactId>
					<version>3.0</version>
				</plugin>
			</plugins>
		</pluginManagement>
```
to ensure that the Spring Boot Maven Plugin would use a recent Java compiler (my build was previously using version 2.0.2, which defaulted to Java 1.3).

I updated the Spring Boot and Spring Framework versions to the latest GA releases.

I also added Maven properties for Akka and Spring versions to consolidated duplicated version numbers in project coordinates.