Add to gradle build pre-requisites check (#1156)
Add to gradle build pre-requisites check (#1156)

* Add to gradle build pre-requisites check
* Link pointing to 'Building Picard'

Due to the usage of the git repository to get the version, Picard fails
to build if the zipped source is downloaded. Based on the same strategy
as GATK, this commit adds a check for the build pre-requisites.
alecw/picard,broadinstitute/picard,broadinstitute/picard,broadinstitute/picard,alecw/picard,alecw/picard,broadinstitute/picard,broadinstitute/picard,alecw/picard