fix(travis): update for breaking change to travis-cargo
fix(travis): update for breaking change to travis-cargo

I'm changing how travis-cargo works, and doing this requires a breaking change. http://huonw.github.io/blog/2015/05/travis-on-the-train-part-2/

lzpfmh/nickel.rs,BenKnigge/nickel.rs,Manishearth/nickel.rs,SimonPersson/nickel.rs,tempbottle/nickel.rs,Ye-Yong-Chi/nickel.rs,cburgdorf/nickel.rs,anvie/nickel.rs,ixjf/nickel.rs,nickel-org/nickel.rs,jolhoeft/nickel.rs,derdela/nickel.rs,Ryman/nickel.rs,nikomatsakis/nickel.rs