update gatk to 4.alpha.2-272-gf4d928f-20170517.201327-1 (#1052)
update gatk to 4.alpha.2-272-gf4d928f-20170517.201327-1 (#1052)

* updating gatk version to 4.alpha.2-272-gf4d928f-20170517.201327-1 
* this includes an important bug fix for GenotypeGVCFs
broadinstitute/gatk-protected,broadinstitute/gatk-protected,broadinstitute/gatk-protected,broadinstitute/gatk-protected,broadinstitute/gatk-protected,broadinstitute/gatk-protected