Add explicit hamcrest-core dependency

Add an explicit dependency to `hamcrest-core` in the
`spring-boot-starter-test` POM. This prevents version 1.1 from
accidentally being pulled in via junit.

Fixes gh-810

spring-boot-dependencies/pom.xml

@@ -303,6 +303,11 @@
 				<artifactId>jetty-annotations</artifactId>
 				<version>${jetty.version}</version>
 			</dependency>
+			<dependency>
+				<groupId>org.hamcrest</groupId>
+				<artifactId>hamcrest-core</artifactId>
+				<version>${hamcrest.version}</version>
+			</dependency>
 			<dependency>
 				<groupId>org.hamcrest</groupId>
 				<artifactId>hamcrest-library</artifactId>

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

@@ -26,6 +26,10 @@
 			<groupId>org.mockito</groupId>
 			<artifactId>mockito-core</artifactId>
 		</dependency>
+		<dependency>
+			<groupId>org.hamcrest</groupId>
+			<artifactId>hamcrest-core</artifactId>
+		</dependency>
 		<dependency>
 			<groupId>org.hamcrest</groupId>
 			<artifactId>hamcrest-library</artifactId>