CI: reduce number of travis CI runs
CI: reduce number of travis CI runs

It's okey to skip current stable Rust, if we are testing both
nightly and older stable.

Undin/intellij-rust,anton-okolelov/intellij-rust,intellij-rust/intellij-rust,intellij-rust/intellij-rust,intellij-rust/intellij-rust,anton-okolelov/intellij-rust,intellij-rust/intellij-rust,intellij-rust/intellij-rust,anton-okolelov/intellij-rust,Undin/intellij-rust,Undin/intellij-rust,anton-okolelov/intellij-rust,intellij-rust/intellij-rust,Undin/intellij-rust,Undin/intellij-rust,Undin/intellij-rust,anton-okolelov/intellij-rust