Bump the default ivy bootstrap jar to 2.4.0.
Bump the default ivy bootstrap jar to 2.4.0.

This has been the stable release for almost a year and pantsbuild/pants
itself needs this version to properly handle the ANDROID_HOME env var
detection it uses in ivysetting.xml.

Previously, the bootstrap was 2.3.0 and the final used by
pantsbuild/pants was 2.4.0 + extras.  For any workspace with a cached
final, the ivysettings.xml would work, but for workspaces w/o an ivy
bootstrapped, the ivysettings.xml would fail.  This change gets
pantsbuild/pants ivysettings.xml working no matter the bootstrap phase.

Testing Done:
First noted the fact that the linux CI was green:
  https://travis-ci.org/pantsbuild/pants/builds/84007338
But the OSX CI was very red:
  https://travis-ci.org/pantsbuild-osx/pants/jobs/84014487

The 2 biggest differences being linux vs OS X and travis-ci cache vs
none respectively.

On my local machine, before the fix, I was able to reproduce the OS X
CI failure mode by deleting my ivy bootstrap cache:
`rm -rf ~/.cache/pants/tools/jvm/ivy/`

After the fix I was no longer able to reproduce, all ran smooth through
bootstrapping.

CI went green here:
  https://travis-ci.org/pantsbuild/pants/builds/84017782

Bugs closed: 2333

Reviewed at https://rbcommons.com/s/twitter/r/2938/

UnrememberMe/pants,wisechengyi/pants,ericzundel/pants,15Dkatz/pants,ericzundel/pants,peiyuwang/pants,jtrobec/pants,wisechengyi/pants,dbentley/pants,15Dkatz/pants,jsirois/pants,jtrobec/pants,UnrememberMe/pants,cevaris/pants,slyphon/pants,pombredanne/pants,gmalmquist/pants,megaserg/pants,cevaris/pants,manasapte/pants,qma/pants,tdyas/pants,lahosken/pants,UnrememberMe/pants,fkorotkov/pants,mateor/pants,mateor/pants,twitter/pants,jessrosenfield/pants,baroquebobcat/pants,lahosken/pants,cevaris/pants,pantsbuild/pants,jessrosenfield/pants,megaserg/pants,twitter/pants,baroquebobcat/pants,wisechengyi/pants,tdyas/pants,megaserg/pants,ericzundel/pants,tdyas/pants,foursquare/pants,wisechengyi/pants,wisechengyi/pants,pantsbuild/pants,landism/pants,fkorotkov/pants,benjyw/pants,fkorotkov/pants,dbentley/pants,dturner-tw/pants,twitter/pants,landism/pants,wisechengyi/pants,baroquebobcat/pants,landism/pants,gmalmquist/pants,manasapte/pants,jessrosenfield/pants,qma/pants,baroquebobcat/pants,UnrememberMe/pants,jtrobec/pants,twitter/pants,pombredanne/pants,jessrosenfield/pants,baroquebobcat/pants,dbentley/pants,megaserg/pants,cevaris/pants,dturner-tw/pants,twitter/pants,ity/pants,ericzundel/pants,benjyw/pants,gmalmquist/pants,qma/pants,jtrobec/pants,peiyuwang/pants,gmalmquist/pants,dbentley/pants,ericzundel/pants,manasapte/pants,mateor/pants,mateor/pants,fkorotkov/pants,fkorotkov/pants,ity/pants,slyphon/pants,gmalmquist/pants,benjyw/pants,fkorotkov/pants,ity/pants,pombredanne/pants,pombredanne/pants,dbentley/pants,pantsbuild/pants,pantsbuild/pants,lahosken/pants,benjyw/pants,mateor/pants,kwlzn/pants,gmalmquist/pants,wisechengyi/pants,jessrosenfield/pants,jessrosenfield/pants,baroquebobcat/pants,pombredanne/pants,jtrobec/pants,jtrobec/pants,baroquebobcat/pants,kwlzn/pants,tdyas/pants,qma/pants,foursquare/pants,fkorotkov/pants,15Dkatz/pants,dbentley/pants,UnrememberMe/pants,peiyuwang/pants,megaserg/pants,peiyuwang/pants,kwlzn/pants,wisechengyi/pants,slyphon/pants,pombredanne/pants,15Dkatz/pants,peiyuwang/pants,ity/pants,mateor/pants,pombredanne/pants,foursquare/pants,jessrosenfield/pants,landism/pants,15Dkatz/pants,cevaris/pants,slyphon/pants,pantsbuild/pants,foursquare/pants,qma/pants,tdyas/pants,lahosken/pants,mateor/pants,tdyas/pants,pantsbuild/pants,ericzundel/pants,qma/pants,mateor/pants,dturner-tw/pants,peiyuwang/pants,dturner-tw/pants,landism/pants,dturner-tw/pants,ity/pants,twitter/pants,ity/pants,jsirois/pants,twitter/pants,foursquare/pants,manasapte/pants,landism/pants,slyphon/pants,manasapte/pants,15Dkatz/pants,twitter/pants,slyphon/pants,ericzundel/pants,megaserg/pants,dturner-tw/pants,baroquebobcat/pants,15Dkatz/pants,UnrememberMe/pants,lahosken/pants,foursquare/pants,tdyas/pants,15Dkatz/pants,ericzundel/pants,benjyw/pants,UnrememberMe/pants,cevaris/pants,lahosken/pants,benjyw/pants,lahosken/pants,cevaris/pants,kwlzn/pants,ity/pants,kwlzn/pants,gmalmquist/pants,UnrememberMe/pants,pantsbuild/pants,wisechengyi/pants,jsirois/pants,jtrobec/pants,foursquare/pants,manasapte/pants,megaserg/pants,dturner-tw/pants,tdyas/pants,baroquebobcat/pants,kwlzn/pants,peiyuwang/pants,manasapte/pants,dbentley/pants,landism/pants,UnrememberMe/pants,foursquare/pants,landism/pants,kwlzn/pants,lahosken/pants,foursquare/pants,slyphon/pants,twitter/pants,tdyas/pants,peiyuwang/pants,benjyw/pants,qma/pants,fkorotkov/pants