travis: more consistent cargo flags (--locked, --release)
travis: more consistent cargo flags (--locked, --release)

graydon/rust,aidancully/rust,aidancully/rust,rust-lang/miri,tsion/miri,GBGamer/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,graydon/rust,rust-lang/miri,aidancully/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,rust-lang/miri,graydon/rust,graydon/rust,graydon/rust,rust-lang/miri,GBGamer/rust,aidancully/rust,graydon/rust