Improve version test (use pkg_resources to compare versions). Also log failures to stderr.
Improve version test (use pkg_resources to compare versions). Also log failures to stderr.

--HG--
branch : pytest-2.7

Carreau/pytest,userzimmermann/pytest,nicoddemus/pytest,chillbear/pytest,lukas-bednar/pytest,mbirtwell/pytest,ddboline/pytest,lukas-bednar/pytest,tgoodlet/pytest,ojake/pytest,hpk42/pytest,icemac/pytest,ojake/pytest,wfxiang08/pytest,JonathonSonesen/pytest,rouge8/pytest,MichaelAquilina/pytest,abusalimov/pytest,davidszotten/pytest,inirudebwoy/pytest,Bjwebb/pytest,The-Compiler/pytest,doordash/pytest,Akasurde/pytest,gabrielcnr/pytest,omarkohl/pytest,Bachmann1234/pytest,alfredodeza/pytest,userzimmermann/pytest,vodik/pytest,ropez/pytest,markshao/pytest,ionelmc/pytest,oleg-alexandrov/pytest,hpk42/pytest,hunse/pytest,vodik/pytest,nicoddemus/pytest,Haibo-Wang-ORG/pytest,ericdill/pytest,untitaker/pytest,JonathonSonesen/pytest,chiller/pytest,txomon/pytest,hunse/pytest,etataurov/pytest,chiller/pytest,bubenkoff/pytest,Carreau/pytest,mhils/pytest,rouge8/pytest,nicoddemus/repo-test,bukzor/pytest,jb098/pytest,takluyver/pytest,ropez/pytest,vmalloc/dessert,pfctdayelise/pytest,ionelmc/pytest,bubenkoff/pytest,rmfitzpatrick/pytest,codewarrior0/pytest,bukzor/pytest,mdboom/pytest,doordash/pytest,ghostsquad/pytest,tareqalayan/pytest,tomviner/pytest,Bachmann1234/pytest,icemac/pytest,mbirtwell/pytest,pytest-dev/pytest,ericdill/pytest,codewarrior0/pytest,MengJueM/pytest,inirudebwoy/pytest,takluyver/pytest,eli-b/pytest,The-Compiler/pytest,tomviner/pytest,omarkohl/pytest,nicoddemus/repo-test,oleg-alexandrov/pytest,Haibo-Wang-ORG/pytest,mdboom/pytest,pelme/pytest,chillbear/pytest,pelme/pytest,jb098/pytest,wfxiang08/pytest,gabrielcnr/pytest,hackebrot/pytest,abusalimov/pytest,flub/pytest,malinoff/pytest,Bjwebb/pytest,skylarjhdownes/pytest,ghostsquad/pytest,jaraco/pytest,MengJueM/pytest,mhils/pytest,untitaker/pytest,RonnyPfannschmidt/pytest