Since SNAPSHOTs are a Maven concept, they aren't treated as anything special in an 'ivy' repository. Unfortunately, there's not (yet) any way to specify a custom pattern for a 'maven' repository, so you're a bit stuck.
The best way to tell Gradle to check for updated versions of a dependency is to flag the dependency as 'changing'. Gradle will then check for updates every 24 hours, but this can be configured using the resolutionStrategy DSL.
// check for updates every build
resolutionStrategy.cacheChangingModulesFor 0, 'seconds'
compile group: "group", name: "projectA", version: "1.1-SNAPSHOT", changing: true