add a couple more Rust versions to CI

.travis.yml

@@ -1,6 +1,8 @@
 language: rust
 rust:
   - 1.8.0
+  - 1.15.0
+  - 1.20.0
   - stable
   - beta
   - nightly

ci/rustup.sh

@@ -5,7 +5,7 @@
 set -ex
 
 export TRAVIS_RUST_VERSION
-for TRAVIS_RUST_VERSION in 1.8.0 stable beta nightly; do
+for TRAVIS_RUST_VERSION in 1.8.0 1.15.0 1.20.0 stable beta nightly; do
     run="rustup run $TRAVIS_RUST_VERSION"
     $run $PWD/ci/test_full.sh
 done