Include plexus-utils as runtimeOnly dependency

[xyloman]

If plexus-utils is not included a null pointer exception is thrown by CommonsExecWebServerFactoryBean.

java.lang.NullPointerException: Cannot invoke "org.eclipse.aether.RepositorySystem.newLocalRepositoryManager(org.eclipse.aether.RepositorySystemSession, org.eclipse.aether.repository.LocalRepository)" because "system" is null
	at app//org.springframework.experimental.boot.server.exec.MavenClasspathEntry.newRepositorySystemSession(MavenClasspathEntry.java:239)
	at app//org.springframework.experimental.boot.server.exec.MavenClasspathEntry.resolve(MavenClasspathEntry.java:164)
	at app//org.springframework.experimental.boot.server.exec.ClasspathBuilder.lambda$build$0(ClasspathBuilder.java:55)
	at [email protected]/java.util.stream.ReferencePipeline$7$1.accept(ReferencePipeline.java:273)
	at [email protected]/java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1708)
	at [email protected]/java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:509)
	at [email protected]/java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:499)
	at [email protected]/java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:921)
	at [email protected]/java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
	at [email protected]/java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:682)
	at app//org.springframework.experimental.boot.server.exec.ClasspathBuilder.build(ClasspathBuilder.java:56)
	at app//org.springframework.experimental.boot.server.exec.CommonsExecWebServerFactoryBean.build(CommonsExecWebServerFactoryBean.java:160)
	at app//org.springframework.experimental.boot.server.exec.CommonsExecWebServerFactoryBean.getObject(CommonsExecWebServerFactoryBean.java:183)
	at app//org.springframework.experimental.boot.server.exec.CommonsExecWebServerFactoryBean.getObject(CommonsExecWebServerFactoryBean.java:47)

[rwinch]

Thanks @xyloman! Can you please provide a test that reproduces this issue so we can ensure that it stays fixed?