use better external check system (#2786)
use better external check system (#2786)

* use better external check system
* Moves Travis to only Linux builds; no more OSX
Jawshy/twofactorauth,bjorno/twofactorauth,rugk/twofactorauth,bjorno/twofactorauth,rj175/twofactorauth,rugk/twofactorauth,jdavis/twofactorauth,vgarmash/twofactorauth,Jawshy/twofactorauth,cez81/twofactorauth,rj175/twofactorauth,jdavis/twofactorauth,bceasydns/twofactorauth,bceasydns/twofactorauth,bjorno/twofactorauth,Jawshy/twofactorauth,Nitrokey/dongleauth,Jawshy/twofactorauth,vgarmash/twofactorauth,cez81/twofactorauth,NormandLemay/twofactorauth,cez81/twofactorauth,bceasydns/twofactorauth,rj175/twofactorauth,Nitrokey/dongleauth,Nitrokey/dongleauth,vgarmash/twofactorauth,rugk/twofactorauth,NormandLemay/twofactorauth,NormandLemay/twofactorauth,jdavis/twofactorauth