ci: tweak travis code coverage configuration

* It appears that the ICE reported in rust-lang/rust#47309 as regressed
into the latest stable version. This change moves coverage collection to
use stable 1.20.0 instead
* Also run code coverage in a dedicated VM instead of a Docker container
as a workaround for travis-ci/travis-ci#9061

.travis.yml

@@ -14,17 +14,16 @@ addons:
 
 matrix:
   include:
-    - rust: nightly
-
-    - rust: stable # Running kcov on stable due to an ICE on nightly: https://github.com/rust-lang/rust/issues/47309
+    - rust: 1.20.0 # Running kcov on (older) stable due to an ICE: https://github.com/rust-lang/rust/issues/47309
       sudo: required # Work around for travis-ci/travis-ci#9061
       env:
         - RUSTFLAGS="-C link-dead-code" # Enable better code coverage at the cost of binary size
       after_success:
         - ./ci/install-kcov.sh && ./ci/coverage.sh
 
+    - rust: nightly
     - rust: beta
-    - rust: 1.20.0
+    - rust: stable
     - os: osx
       rust: stable