Add maven.compiler.source and target properties

Add `maven.compiler.source` and `maven.compiler.target` properties to
the spring-boot-starter-parent to allow easier customization of the
maven compiler. Both properties default to `${java.version}`.

Fixes gh-1684

spring-boot-starters/spring-boot-starter-parent/pom.xml

@@ -20,6 +20,8 @@
 		<java.version>1.6</java.version>
 		<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
 		<project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
+		<maven.compiler.source>${java.version}</maven.compiler.source>
+		<maven.compiler.target>${java.version}</maven.compiler.target>
 	</properties>
 	<dependencyManagement>
 		<dependencies>
@@ -62,8 +64,8 @@
 					<groupId>org.apache.maven.plugins</groupId>
 					<artifactId>maven-compiler-plugin</artifactId>
 					<configuration>
-						<source>${java.version}</source>
-						<target>${java.version}</target>
+						<source>${maven.compiler.source}</source>
+						<target>${maven.compiler.target}</target>
 					</configuration>
 				</plugin>
 				<plugin>