ci: Allow skipping tests when running under CI

This adds `--cfg=travis` to builds under CI, allowing tests to be
conditionally ignored by marking them with

    #[cfg_attr(travis, ignore)]

refs #568

Author: kamalmarhubi

CONTRIBUTING.md

@@ -85,6 +85,13 @@ locally.  More information is available in the [CI Readme][ci-readme].
 [travis-ci]: https://travis-ci.org/nix-rust/nix
 [ci-readme]: ci/README.md
 
+### Disabling a test in the CI environment
+
+Sometimes there are features that cannot be tested in the CI environment.
+To stop a test from running under CI, add `#[cfg_attr(travis, ignore)]`
+to it. Please include a comment describing the reason it shouldn't run
+under CI, and a link to an upstream issue if possible!
+
 ## bors, the bot who merges all the PRs
 
 All pull requests are merged via [bors], an integration bot. After the

ci/script.sh

@@ -3,6 +3,11 @@
 set -ex
 
 main() {
+    # Add a cfg spec to allow disabling specific tests under CI.
+    if [ "$TRAVIS" = true ]; then
+        export RUSTFLAGS=--cfg=travis
+    fi
+
     # Build debug and release targets
     cross build --target $TARGET
     cross build --target $TARGET --release