Consistent Thymeleaf Version in Boot Samples

Issue gh-4802

samples/boot/helloworld/spring-security-samples-boot-helloworld.gradle

@@ -1,5 +1,7 @@
 apply plugin: 'io.spring.convention.spring-sample-boot'
 
+ext['thymeleaf.version'] = '3.0.9.RELEASE'
+
 dependencies {
 	compile project(':spring-security-config')
 	compile project(':spring-security-web')

samples/boot/insecure/spring-security-samples-boot-insecure.gradle

@@ -1,5 +1,7 @@
 apply plugin: 'io.spring.convention.spring-sample-boot'
 
+ext['thymeleaf.version'] = '3.0.9.RELEASE'
+
 dependencies {
 	compile 'org.springframework.boot:spring-boot-starter-thymeleaf'
 	compile 'org.springframework.boot:spring-boot-starter-web'

samples/boot/oauth2login/spring-security-samples-boot-oauth2login.gradle

@@ -1,5 +1,7 @@
 apply plugin: 'io.spring.convention.spring-sample-boot'
 
+ext['thymeleaf.version'] = '3.0.9.RELEASE'
+
 dependencies {
 	compile project(':spring-security-config')
 	compile project(':spring-security-oauth2-client')