Upgrade Mockito from 1.10.19 -> 2.10.0.

[cmnbroad]

I've had several Travis test failures (on my picard removal branch) that appear to be failures during kryo serialization of a mocked ReferenceMultiSource object (based on the failing class name, (org.broadinstitute.hellbender.engine.datasources.ReferenceMultiSource$$EnhancerByMockitoWithCGLIB$$b0dc631f, which looks like the CGLIB names mentioned here).

We're on an ancient version of mockito anyway, and newer versions no longer use cglib, so it seemed like a good time to upgrade. To do so I also had to replace usage of the method getArgumentAt, which has been deprecated in favor of getArgument.

[codecov-io]

Codecov Report

Merging #3581 into master will increase coverage by 0.007%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##              master     #3581       +/-   ##
===============================================
+ Coverage     79.937%   79.944%   +0.007%     
- Complexity     18005     18009        +4     
===============================================
  Files           1206      1206               
  Lines          65578     65607       +29     
  Branches       10245     10249        +4     
===============================================
+ Hits           52421     52449       +28     
  Misses          9061      9061               
- Partials        4096      4097        +1

Impacted Files	Coverage Δ	Complexity Δ
...nder/tools/spark/pathseq/PathSeqPipelineSpark.java	90% <0%> (+3.115%)	11% <0%> (+4%)	⬆️

[cmnbroad]

Note: it looks like this doesn't fix the ConcurrentModificationException during serialization while running the unit tests on my Picard branch.

[lbergelson]

@cmnbroad It's unfortunate that this doesn't fix your problem. We might as well merge this anyway though since moving to a modern mockito seems like a good idea.