Update xdist link; BitBucket -> GitHub
Update xdist link; BitBucket -> GitHub

xdist is now hosted on GitHub.
tgoodlet/pytest,vmalloc/dessert,JonathonSonesen/pytest,The-Compiler/pytest,markshao/pytest,skylarjhdownes/pytest,RonnyPfannschmidt/pytest,nicoddemus/pytest,jaraco/pytest,malinoff/pytest,txomon/pytest,userzimmermann/pytest,davidszotten/pytest,MengJueM/pytest,hpk42/pytest,hackebrot/pytest,MichaelAquilina/pytest,omarkohl/pytest,Bachmann1234/pytest,userzimmermann/pytest,tomviner/pytest,ddboline/pytest,rmfitzpatrick/pytest,alfredodeza/pytest,lukas-bednar/pytest,flub/pytest,Bachmann1234/pytest,omarkohl/pytest,vodik/pytest,MengJueM/pytest,Akasurde/pytest,ropez/pytest,lukas-bednar/pytest,tareqalayan/pytest,The-Compiler/pytest,eli-b/pytest,pytest-dev/pytest,nicoddemus/pytest,etataurov/pytest,vodik/pytest,pfctdayelise/pytest,JonathonSonesen/pytest,ropez/pytest,tomviner/pytest,hpk42/pytest