Use travis conditionals to shrink the matrix
Use travis conditionals to shrink the matrix

Ensures that we shouldn't schedule unnecessary builds on PRs/master

rust-lang/rustup,inejge/rustup.rs,nodakai/rustup.rs,nodakai/rustup.rs,inejge/rustup.rs,nodakai/rustup.rs,nodakai/rustup.rs,rust-lang/rustup,rust-lang/rustup,inejge/rustup.rs,rust-lang/rustup,inejge/rustup.rs,rust-lang/rustup,nodakai/rustup.rs,inejge/rustup.rs