Cleanup Travis config

* use explicit Ruby version numbers
* bundler cache works with Ruby, Cargo cache needs to be explicit
* build using stable Rust

.travis.yml

@@ -1,16 +1,19 @@
-sudo: true
+sudo: false
 language: ruby
 os:
 - linux
 - osx
 cache:
-- bundler
-- cargo
+  bundler: true
+  directories:
+  - $HOME/.cargo
+  - $TRAVIS_BUILD_DIR/target
 rvm:
-- 2.2
-- 2.3
+- 2.2.5
+- 2.3.3
 env:
   global:
+  - PATH=$PATH:$HOME/.cargo/bin
   - secure: 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
   matrix:
   - NO_ENV_VARS=USED
@@ -22,7 +25,4 @@ matrix:
   - env: WITH_REGRESSION=true
   - env: TEST_MONKEYPATCHES=true WITH_REGRESSION=true
 before_install:
-- gem install bundler -v 1.14.5
-- curl -sSf https://static.rust-lang.org/rustup.sh | sudo sh -s -- --channel=nightly
-- bundle && bundle exec rake build_lib
-script: bundle exec rake test
+- curl -sSf https://sh.rustup.rs | sh -s -- -y --default-toolchain stable