chore(travis): no nightly

To make nightly tests work, one will have to do some additional
scripting to call it with the correct features (see makefile).

For now, building it on stable is just fine I think.

.travis.yml

@@ -2,7 +2,6 @@ language: rust
 sudo: required
 rust:
 - stable
-- nightly
 before_script:
 - pip install 'travis-cargo<0.2' --user && export PATH=$HOME/.local/bin:$PATH
 script: