TRAVIS: install gtk-sharp2, but not nunit
TRAVIS: install gtk-sharp2, but not nunit

I got the gtk-sharp package name wrong.

Also, since nunit is already checked in under tools, we don't need to install it.

Jaykul/Pash,ForNeVeR/Pash,JayBazuzi/Pash,Jaykul/Pash,ForNeVeR/Pash,Jaykul/Pash,sburnicki/Pash,sburnicki/Pash,WimObiwan/Pash,mrward/Pash,Jaykul/Pash,sillvan/Pash,sillvan/Pash,mrward/Pash,sillvan/Pash,JayBazuzi/Pash,JayBazuzi/Pash,WimObiwan/Pash,mrward/Pash,JayBazuzi/Pash,WimObiwan/Pash,sillvan/Pash,WimObiwan/Pash,sburnicki/Pash,ForNeVeR/Pash,ForNeVeR/Pash,mrward/Pash,sburnicki/Pash