Adding OpenJDK8 to travis configuration

[jacarey]

Added OpenJDK8 as a build configuration to be tested via Travis.

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 #875 into master will increase coverage by 0.022%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##              master      #875       +/-   ##
===============================================
+ Coverage     65.003%   65.025%   +0.022%     
- Complexity      7231      7233        +2     
===============================================
  Files            528       528               
  Lines          31894     31894               
  Branches        5444      5444               
===============================================
+ Hits           20732     20739        +7     
+ Misses          9016      9011        -5     
+ Partials        2146      2144        -2

Impacted Files	Coverage Δ	Complexity Δ
...dk/samtools/seekablestream/SeekableHTTPStream.java	54.545% <0%> (-1.515%)	9% <0%> (-1%)
...va/htsjdk/samtools/util/AsyncBufferedIterator.java	69.565% <0%> (+1.087%)	17% <0%> (ø)	⬇️
...a/htsjdk/samtools/cram/encoding/rans/Decoding.java	85.714% <0%> (+3.571%)	12% <0%> (+1%)	⬆️
...samtools/util/AsyncBlockCompressedInputStream.java	76% <0%> (+4%)	13% <0%> (+1%)	⬆️
...sjdk/samtools/util/Md5CalculatingOutputStream.java	79.487% <0%> (+7.692%)	9% <0%> (+1%)	⬆️

[pshapiro4broad]

👍

Does it seem odd to you that this change affected coverage?