disable pretty output if we're not running in a valid tty
disable pretty output if we're not running in a valid tty

Summary:
This makes -reporter pretty effectively behave like -reporter plain when
we're not running in a nice terminal (e.g. when we're running on
BuildBot).

Test Plan:
Piped output of xcodetool to `less` and verified that ANSI color codes
weren't used.

Richardlihui/xctool,kernandreas/xctool,leopardpan/xctool,kernandreas/xctool,theiren46/xctool,duk42111/xctool,blighli/xctool,0xced/xctool,AutoScout24/xctool,bgertzfield/xctool,bhamiltoncx/xctool,trello/xctool,kamoljan/xctool,modulexcite/xctool,soundcloud/xctool,blighli/xctool,ACChe/xctool,trello/xctool,ACChe/xctool,githubdelegate/xctool,Monits/xctool,adamjernst/xctool,FiftyThree/xctool,adamjernst/xctool,duk42111/xctool,modulexcite/xctool,bgertzfield/xctool,1yvT0s/xctool,Richardlihui/xctool,facebook/xctool,githubdelegate/xctool,kcoons/xctool,bhamiltoncx/xctool,dvor/xctool,bhamiltoncx/xctool,SuPair/xctool,AutoScout24/xctool,blighli/xctool,1yvT0s/xctool,n054/xctool,modulexcite/xctool,theiren46/xctool,soundcloud/xctool,kcoons/xctool,leopardpan/xctool,bhamiltoncx/xctool,agreco/xctool,duk42111/xctool,AutoScout24/xctool,duk42111/xctool,soundcloud/xctool,FiftyThree/xctool,Monits/xctool,facebook/xctool,lqi/xctool,trello/xctool,PSPDFKit-labs/xctool,bhamiltoncx/xctool,theiren46/xctool,lpios/xctool,bgertzfield/xctool,PSPDFKit-labs/xctool,facebook/xctool,bgertzfield/xctool,ekingo/xctool,bgertzfield/xctool,facebook/xctool,lpios/xctool,adamjernst/xctool,ekingo/xctool,kamoljan/xctool,kernandreas/xctool,leopardpan/xctool,jspahrsummers/xctool,LegNeato/xctool,bhamiltoncx/xctool,0xced/xctool,dvor/xctool,githubdelegate/xctool,Monits/xctool,n054/xctool,ACChe/xctool,LegNeato/xctool,lpios/xctool,yaoxiaoyong/xctool,kernandreas/xctool,Richardlihui/xctool,yaoxiaoyong/xctool,trello/xctool,PSPDFKit-labs/xctool,agreco/xctool,bgertzfield/xctool,Monits/xctool,Richardlihui/xctool,kamoljan/xctool,dvor/xctool,ACChe/xctool,FiftyThree/xctool,FiftyThree/xctool,soundcloud/xctool,SuPair/xctool,ekingo/xctool,SuPair/xctool,AutoScout24/xctool,soundcloud/xctool,lpios/xctool,kcoons/xctool,n054/xctool,ekingo/xctool,facebook/xctool,yaoxiaoyong/xctool,agreco/xctool,kamoljan/xctool,theiren46/xctool,jspahrsummers/xctool,adamjernst/xctool,LegNeato/xctool,dvor/xctool,githubdelegate/xctool,facebook/xctool,jspahrsummers/xctool,LegNeato/xctool,lqi/xctool,blighli/xctool,leopardpan/xctool,SuPair/xctool,agreco/xctool,lqi/xctool,jspahrsummers/xctool,ACChe/xctool,1yvT0s/xctool,1yvT0s/xctool,n054/xctool,PSPDFKit-labs/xctool,lpios/xctool,modulexcite/xctool,yaoxiaoyong/xctool,n054/xctool,PSPDFKit-labs/xctool,ACChe/xctool,soundcloud/xctool,lpios/xctool,n054/xctool