fix(travis): don't fail the build if our min rustc version fails
fix(travis): don't fail the build if our min rustc version fails

This causes some unnecessary blocks for PRs, but it's probably
still worth keeping around as a canary for when things do break.

Pull request: #337
Approved by: Ryman

anvie/nickel.rs,BenKnigge/nickel.rs,nickel-org/nickel.rs,jolhoeft/nickel.rs