Remove obsolete call in merge_variant_headers()

Now that htslib expects us to sync headers manually, we don't
need to pass in a nullptr when we're done adding samples to
a header

Resolves #363
Merge pull request #364 from broadinstitute/dr_remove_obsolete_call_in_merge_variant_headers

Remove obsolete call in merge_variant_headers()
Change m_pending_variants to a vector
Merge pull request #367 from broadinstitute/jt_pending_vector

Change m_pending_variants to a vector
Revert "Make CMAKE the default build script"

This reverts commit ffe7d3bccf7fe3854d695f9df451ad46c54b31e5.
Diversify the ref bases for a slightly better test
Minor edits