Excluding Tomcat dependencies from Spring Boot in Gradle

I'm using Spring Boot with Jetty and it seems I cannot exclude all the Tomcat dependencies in my Gradle build file.

Relevant part of build.gradle:

compile("org.springframework.boot:spring-boot-starter") {
    exclude module: "tomcat-embed-el"
}
compile("org.springframework.boot:spring-boot-starter-jetty")

compile("org.springframework.boot:spring-boot-starter-web") {
    exclude module: "spring-boot-starter-tomcat"
}

Yet when I run gradle dependencies parts of tomcat are still there, and causing issues with WebSockets:

...
|    
+--- org.springframework.boot:spring-boot-starter-web: -> 1.4.1.RELEASE
|    +--- org.springframework.boot:spring-boot-starter:1.4.1.RELEASE (*)
|    +--- org.hibernate:hibernate-validator:5.2.4.Final
|    |    +--- javax.validation:validation-api:1.1.0.Final
|    |    +--- org.jboss.logging:jboss-logging:3.2.1.Final -> 3.3.0.Final
|    |    \--- com.fasterxml:classmate:1.1.0 -> 1.3.1
|    +--- com.fasterxml.jackson.core:jackson-databind:2.8.3
|    |    +--- com.fasterxml.jackson.core:jackson-annotations:2.8.0 -> 2.8.3
|    |    \--- com.fasterxml.jackson.core:jackson-core:2.8.3
|    +--- org.springframework:spring-web:4.3.3.RELEASE
|    |    +--- org.springframework:spring-aop:4.3.3.RELEASE (*)
|    |    +--- org.springframework:spring-beans:4.3.3.RELEASE (*)
|    |    +--- org.springframework:spring-context:4.3.3.RELEASE (*)
|    |    \--- org.springframework:spring-core:4.3.3.RELEASE
|    +--- org.springframework:spring-webmvc:4.3.3.RELEASE
|    |    +--- org.springframework:spring-aop:4.3.3.RELEASE (*)
|    |    +--- org.springframework:spring-beans:4.3.3.RELEASE (*)
|    |    +--- org.springframework:spring-context:4.3.3.RELEASE (*)
|    |    +--- org.springframework:spring-core:4.3.3.RELEASE
|    |    +--- org.springframework:spring-expression:4.3.3.RELEASE (*)
|    |    \--- org.springframework:spring-web:4.3.3.RELEASE (*)
|    \--- org.springframework.boot:spring-boot-starter-tomcat:1.4.1.RELEASE
|         +--- org.apache.tomcat.embed:tomcat-embed-core:8.5.5
|         +--- org.apache.tomcat.embed:tomcat-embed-el:8.5.5
|         \--- org.apache.tomcat.embed:tomcat-embed-websocket:8.5.5
|              \--- org.apache.tomcat.embed:tomcat-embed-core:8.5.5
...

Why isn't spring-boot-starter-tomcat excluded from spring-boot-starter-web ?