Rev htslib to be in sync with our fork, and remove ugly hack from SyncedVariantReader

Hack is no longer needed now that we've patched bcf_sr_add_reader()
in our htslib fork.

Resolves #342
Merge pull request #345 from broadinstitute/dr_rev_htslib_and_remove_synced_reader_hack

Rev htslib to be in sync with our fork, and remove ugly hack from SyncedVariantReader