change Bulk#execute to use promise chaining
change Bulk#execute to use promise chaining

jesperkristensen/jsforce,parthiban019/jsforce,jesperkristensen/jsforce,bhaskarmelkani/jsforce,jsforce/jsforce,bhaskarmelkani/jsforce,jdcrensh/jsforce,jdcrensh/jsforce,jsforce/jsforce,parthiban019/jsforce,saleswise/jsforce,rdietrick/jsforce,saleswise/jsforce,polidog/jsforce,lkatney/jsforce,nutthawan/jsforce,pcon/jsforce,lkatney/jsforce,nutthawan/jsforce,saleswise/jsforce,kimchouard/jsforce,pcon/jsforce,jsforce/jsforce,joeferraro/jsforce,joeferraro/jsforce,stomita/node-salesforce,pcon/jsforce