ci: run the linter from travis in a separate job

Divides the CI jobs by a custom `matrix:`, with a new linter job
using an existing node.js binary to run `make lint-ci` (eslint).

Refs: #71
PR-URL: #75
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

.travis.yml

@@ -1,20 +1,30 @@
 language: cpp
 compiler:
   - clang
-os:
-  - linux
-  - osx
 sudo: false
 cache: ccache
+matrix:
+  include:
+    - os: linux
+      node_js: "8"
+      script:
+        - NODE=$(which node) make lint-ci
+    - os: linux
+      install:
+        - ./configure
+        - make -j2 V=
+      script:
+        - make -j2 test-ci
+    - os: osx
+      install:
+        - ./configure
+        - make -j2 V=
+      script:
+        - make -j2 test-ci
 before_install:
   - export HOMEBREW_NO_AUTO_UPDATE=1 # work around https://github.com/travis-ci/travis-ci/issues/7456
   - if [ $TRAVIS_OS_NAME = osx ]; then brew install ccache; fi
   - export PATH="/usr/local/opt/ccache/libexec:$PATH"
   - export CXX="ccache clang++ -Qunused-arguments"
   - export CC="ccache clang -Qunused-arguments"
   - export JOBS=2
-install:
-  - ./configure
-  - make -j2 V=
-script:
-  - make -j2 test-ci