Update metadata.avdl
Update metadata.avdl

* dateOfBirth to ISO8601/string (already discussed in https://github.com/ga4gh/schemas/pull/328)
dcolligan/schemas,ga4gh/schemas,nlwashington/schemas,dnanexus-rnd/schemas,diekhans/ga4gh-schemas,hjellinek/jeltje-schemas-sphinx,sarahhunt/schemas,buske/schemas,heuermh/ga4gh-schemas,benedictpaten/schemas,buske/schemas,benedictpaten/schemas,macieksmuga/schemas,pcingola/schemas,lh3/schemas,Jeltje/ga4gh-schemas,saupchurch/schemas,hjellinek/schemas,laserson/schemas,laserson/schemas,dglazer/schemas,ga4gh/ga4gh-schemas,sarahhunt/schemas,Jeltje/ga4gh-schemas,adamnovak/schemas,nlwashington/schemas,adamnovak/schemas,dnanexus-rnd/schemas,lh3/schemas,diekhans/ga4gh-schemas,hjellinek/schemas