Improve Gradle dependencies version management

This commit also updates to the following dependencies:
 - Spring Framework 4.2.3.RELEASE
 - RxJava 1.1.0

spring-web-reactive/build.gradle

@@ -30,37 +30,38 @@ configurations.all {
 	resolutionStrategy.cacheChangingModulesFor 0, 'seconds'
 }
 
+ext {
+	springVersion = '4.2.3.RELEASE'
+	reactorVersion = '2.1.0.BUILD-SNAPSHOT'
+	tomcatVersion = '8.0.28'
+	jettyVersion = '9.3.5.v20151012'
+}
+
 dependencies {
-	compile "org.springframework:spring-core:4.2.2.RELEASE"
-	compile "org.springframework:spring-web:4.2.2.RELEASE"
+	compile "org.springframework:spring-core:${springVersion}"
+	compile "org.springframework:spring-web:${springVersion}"
 	compile "org.reactivestreams:reactive-streams:1.0.0"
-	compile "io.projectreactor:reactor-core:2.1.0.BUILD-SNAPSHOT"
+	compile "io.projectreactor:reactor-core:${reactorVersion}"
 	compile "commons-logging:commons-logging:1.2"
 
-	optional "com.fasterxml.jackson.core:jackson-databind:2.6.2"
-
+	optional 'io.reactivex:rxjava:1.1.0'
 	optional "io.reactivex:rxnetty:0.5.0-SNAPSHOT"
-
-	optional "io.projectreactor:reactor-stream:2.1.0.BUILD-SNAPSHOT"
-	optional "io.projectreactor:reactor-net:2.1.0.BUILD-SNAPSHOT"
-
-	optional 'org.apache.tomcat:tomcat-util:8.0.28'
-	optional 'org.apache.tomcat.embed:tomcat-embed-core:8.0.28'
-
+	optional "com.fasterxml.jackson.core:jackson-databind:2.6.2"
+	optional "io.projectreactor:reactor-stream:${reactorVersion}"
+	optional "io.projectreactor:reactor-net:${reactorVersion}"
+	optional "org.apache.tomcat:tomcat-util:${tomcatVersion}"
+	optional "org.apache.tomcat.embed:tomcat-embed-core:${tomcatVersion}"
 	optional 'io.undertow:undertow-core:1.3.5.Final'
-
-	optional 'org.eclipse.jetty:jetty-server:9.3.5.v20151012'
-	optional 'org.eclipse.jetty:jetty-servlet:9.3.5.v20151012'
+	optional "org.eclipse.jetty:jetty-server:${jettyVersion}"
+	optional "org.eclipse.jetty:jetty-servlet:${jettyVersion}"
 
 	provided "javax.servlet:javax.servlet-api:3.1.0"
 
 	testCompile "junit:junit:4.12"
-	testCompile "org.springframework:spring-test:4.2.2.RELEASE"
-
+	testCompile "org.springframework:spring-test:${springVersion}"
 	testCompile "org.slf4j:slf4j-jcl:1.7.12"
 	testCompile "org.slf4j:jul-to-slf4j:1.7.12"
 	testCompile("log4j:log4j:1.2.16")
-
 	testCompile("org.mockito:mockito-core:1.10.19") {
 		exclude group: 'org.hamcrest', module: 'hamcrest-core'
 	}