Avoid ValueError exception from a reporting thread on shutdown
Avoid ValueError exception from a reporting thread on shutdown

Fixes a problem seen in Ubuntu 14.0.4 under python 2.7.6 frequently when invoking pants:

```
./pants -V
0.0.65
Exception in thread output-emitter (most likely raised during interpreter shutdown):
Traceback (most recent call last):
  File "/usr/lib/python2.7/threading.py", line 810, in __bootstrap_inner
  File "/home/zundel/.cache/pants/setup/bootstrap/pants.Ob3x3U/install/local/lib/python2.7/site-packages/pants/reporting/report.py", line 23, in run
  File "/usr/lib/python2.7/threading.py", line 620, in wait
  File "/usr/lib/python2.7/threading.py", line 363, in wait
<type 'exceptions.ValueError'>: list.remove(x): x not in list
```

This message was harmless, but annoying, probably due to some bug in the threading library.

Testing Done:
Manual testing.
CI passed at https://travis-ci.org/pantsbuild/pants/builds/98110978

Bugs closed: 2750, 2751

Reviewed at https://rbcommons.com/s/twitter/r/3278/

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