Create a basic Grails project and then install the plugin spock 1.5-groovy-1.7-SNAPSHOT. STS will complain that it can't find the class JUnitResultFormatter. This can be rectified by adding ant-junit-1.7.1.jar to the build path, but this library is provided by Grails so should probably be included on the classpath by default.
Note that the workaround will result in a bunch of other errors appearing. This is because the plugin's dependencies are declared in build scope, but they should probably be declared in test scope instead. I'll deal with that separately.