Default value for build (37) is always used in maf file

[holmeso]

Describe the bug
The NCBI_Build column in the mafs generated by qannotate's Vcf2Maf class use the default value of the MafElement.NCBI_Build enum element, which is set to "37"

This needs to change so that the actual build is used (ie. 38 if applicable).

https://docs.gdc.cancer.gov/Data/File_Formats/MAF_Format/

To Reproduce
Steps to reproduce the behavior:
Run qannotate's Vcf2Maf class and examine the output

Expected behavior
The NCBI_Build field should contain the build that was used to map the BAMs