Bump up curator version to 2.7.1

<!-- markdown --> Apache Curator version change: Apache Hadoop has updated the version of Apache Curator used from 2.6.0 to 2.7.1. This change should be binary and source compatible for the majority of downstream users. Notable exceptions: * Binary incompatible change: org.apache.curator.utils.PathUtils.validatePath(String) changed return types. Downstream users of this method will need to recompile. * Source incompatible change: org.apache.curator.framework.recipes.shared.SharedCountReader added a method to its interface definition. Downstream users with custom implementations of this interface can continue without binary compatibility problems but will need to modify their source code to recompile. * Source incompatible change: org.apache.curator.framework.recipes.shared.SharedValueReader added a method to its interface definition. Downstream users with custom implementations of this interface can continue without binary compatibility problems but will need to modify their source code to recompile. Downstream users are reminded that while the Hadoop community will attempt to avoid egregious incompatible dependency changes, there is currently no policy around when Hadoop's exposed dependencies will change across versions (ref http://hadoop.apache.org/docs/current/hadoop-project-dist/hadoop-common/Compatibility.html#Java_Classpath ).