Changing htsjdk to produce sam version 1.6

[lbergelson]

• Htsjdk has techinically been producing Sam version 1.6 since support for long cigars was added.
• Updating that list of acceptable versions to include 1.6 and setting the header version of new bams to 1.6

Checklist

• Code compiles correctly
• New tests covering changes and new functionality
• All tests passing
• Extended the README / documentation, if necessary
• Is not backward compatible (breaks binary or source compatibility)

[codecov-io]

Codecov Report

Merging #1211 into master will increase coverage by 0.095%.
The diff coverage is 100%.

@@               Coverage Diff               @@
##              master     #1211       +/-   ##
===============================================
+ Coverage     68.702%   68.796%   +0.095%     
- Complexity      8061      8094       +33     
===============================================
  Files            542       542               
  Lines          32711     32887      +176     
  Branches        5533      5577       +44     
===============================================
+ Hits           22473     22625      +152     
- Misses          8038      8054       +16     
- Partials        2200      2208        +8

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/htsjdk/samtools/SAMFileHeader.java	64.623% <100%> (-3.641%)	54 <1> (+9)
...samtools/util/AsyncBlockCompressedInputStream.java	72% <0%> (-4%)	12% <0%> (-1%)
...va/htsjdk/samtools/sra/SRAIndexedSequenceFile.java	62.162% <0%> (-2.703%)	6% <0%> (-1%)
.../main/java/htsjdk/samtools/SAMTextHeaderCodec.java	87.668% <0%> (+6.263%)	72% <0%> (+25%)	⬆️
...sjdk/samtools/util/Md5CalculatingOutputStream.java	74.359% <0%> (+7.692%)	8% <0%> (+1%)	⬆️

[cmnbroad]

The changes themselves look fine. The only question I have is whether there are any special cases we need to consider when merging old headers with 1.6 headers ? Otherwise LGTM.

[cmnbroad]

👍 to taking this and addressing #1202 separately.