updated .travis.yml to limit the automated build to nightly branch.
updated .travis.yml to limit the automated build to nightly branch.

nox/rust-encoding,jedisct1/rust-encoding,tempbottle/rust-encoding,lifthrasiir/rust-encoding,lifthrasiir/rust-encoding,nox/rust-encoding,servo/rust-encoding,jedisct1/rust-encoding,servo/rust-encoding,lifthrasiir/rust-encoding,nox/rust-encoding,tempbottle/rust-encoding,tempbottle/rust-encoding