Use Gradle wrapper when building BWC
Use Gradle wrapper when building BWC

This commit modifies the BWC build to invoke the Gradle wrapper. The
motivation for this is two-fold:
 - BWC versions might be dependent on a different version of Gradle than
   the current version of Gradle
 - in a follow-up we are going to need to be able to set JAVA_HOME to a
   different value than the current value of JAVA_HOME

Relates elastic/x-pack-elasticsearch#3502

Original commit: elastic/x-pack-elasticsearch@48bb737a8f73c5588cce8b64e783592de2c61c86

vroyer/elassandra,vroyer/elassandra,vroyer/elassandra,strapdata/elassandra,strapdata/elassandra,strapdata/elassandra,strapdata/elassandra,strapdata/elassandra