Increase Gradle heap space to 1536m
Increase Gradle heap space to 1536m

This commit increases the heap space for the Gradle processes to
1536m. Note that the default is 1024m.

Original commit: elastic/x-pack-elasticsearch@3b5cdc30b2297eba151f9c776366669ef9a71b10

coding0011/elasticsearch,uschindler/elasticsearch,gfyoung/elasticsearch,HonzaKral/elasticsearch,uschindler/elasticsearch,nknize/elasticsearch,scorpionvicky/elasticsearch,coding0011/elasticsearch,gingerwizard/elasticsearch,scorpionvicky/elasticsearch,scorpionvicky/elasticsearch,robin13/elasticsearch,uschindler/elasticsearch,HonzaKral/elasticsearch,nknize/elasticsearch,uschindler/elasticsearch,coding0011/elasticsearch,gingerwizard/elasticsearch,GlenRSmith/elasticsearch,robin13/elasticsearch,gfyoung/elasticsearch,GlenRSmith/elasticsearch,coding0011/elasticsearch,gfyoung/elasticsearch,nknize/elasticsearch,robin13/elasticsearch,gingerwizard/elasticsearch,scorpionvicky/elasticsearch,gingerwizard/elasticsearch,gingerwizard/elasticsearch,scorpionvicky/elasticsearch,robin13/elasticsearch,gingerwizard/elasticsearch,GlenRSmith/elasticsearch,HonzaKral/elasticsearch,gfyoung/elasticsearch,GlenRSmith/elasticsearch,gingerwizard/elasticsearch,nknize/elasticsearch,GlenRSmith/elasticsearch,HonzaKral/elasticsearch,nknize/elasticsearch,gfyoung/elasticsearch,coding0011/elasticsearch,uschindler/elasticsearch,robin13/elasticsearch