csl sbt build: Use git rev-parse instead of symbolic-ref
csl sbt build: Use git rev-parse instead of symbolic-ref

Problem

sbt build calls 'git symbolic-ref' which fails in a detached HEAD state. This breaks github travis builds, e.g. https://travis-ci.org/twitter/util/jobs/52793532

Solution

Use 'git rev-parse --abbrev HEAD' instead.

RB_ID=596030

BuoyantIO/twitter-util,tdyas/util,twitter/util,nkhuyu/util,travisbrown/util,cogitate/twitter-util-uuid,stremlenye/util,travisbrown/util,hydro2k/util,Jiri-Kremser/util,wuqiangxjtu/util,tenghuihua/util,hydro2k/util,travisbrown/util,tenghuihua/util,tenghuihua/util,BuoyantIO/twitter-util,cogitate/twitter-util-uuid,slackhappy/util,edombowsky/util,hydro2k/util,travisbrown/util,RUNDSP/util,wuqiangxjtu/util,RUNDSP/util,folone/util,slackhappy/util,RUNDSP/util,folone/util,LitmusHealth/twitter-utils,tenghuihua/util,folone/util,RUNDSP/util,stremlenye/util,Jiri-Kremser/util,LitmusHealth/twitter-utils,nkhuyu/util,edombowsky/util,twitter/util,tdyas/util,wuqiangxjtu/util,Jiri-Kremser/util,edombowsky/util,hydro2k/util,edombowsky/util,slackhappy/util,slackhappy/util,tdyas/util,cogitate/twitter-util-uuid,tdyas/util,nkhuyu/util,stremlenye/util,folone/util,BuoyantIO/twitter-util,LitmusHealth/twitter-utils,cogitate/twitter-util-uuid,twitter/util,wuqiangxjtu/util,nkhuyu/util,BuoyantIO/twitter-util,Jiri-Kremser/util,stremlenye/util,LitmusHealth/twitter-utils