add electron to travis test matrix (#848)

* add electron to travis test matrix

* fix mac build

* add electon 5.0.1

* bump electron versions

.travis.yml

@@ -22,6 +22,10 @@ env:
   - TRAVIS_NODE_VERSION="10"
   - TRAVIS_NODE_VERSION="11"
   - TRAVIS_NODE_VERSION="12"
+  - TRAVIS_NODE_VERSION="lts/*" ELECTRON_VERSION="2.0.18"
+  - TRAVIS_NODE_VERSION="lts/*" ELECTRON_VERSION="3.1.9"
+  - TRAVIS_NODE_VERSION="lts/*" ELECTRON_VERSION="4.2.0"
+  - TRAVIS_NODE_VERSION="lts/*" ELECTRON_VERSION="5.0.1"
 matrix:
   exclude:
     - os: osx
@@ -31,6 +35,7 @@ install:
   - if [[ $TRAVIS_OS_NAME == "linux" ]]; then export CXX=g++-4.8; fi
   - $CXX --version
   - if [[ $TRAVIS_NODE_VERSION == "0.8" ]]; then npm config set strict-ssl false; fi
-  - if [[ $(echo "$TRAVIS_NODE_VERSION < 4" | bc -l) ]]; then npm install npm@2 && mv node_modules npm && npm/.bin/npm --version && npm/.bin/npm install; else npm --version && npm install; fi
-  - node_modules/.bin/node-gyp rebuild --directory test
-script: node_modules/.bin/tap --gc test/js/*-test.js
+  - if [[ -z "$ELECTRON_VERSION" && $(echo "$TRAVIS_NODE_VERSION < 4" | bc -l)  == "1" ]]; then npm install npm@2 && mv node_modules npm && npm/.bin/npm --version && npm/.bin/npm install; else npm --version && npm install; fi
+  - if [[ -z "$ELECTRON_VERSION" ]]; then node_modules/.bin/node-gyp rebuild --directory test; else node_modules/.bin/node-gyp rebuild --target=v$ELECTRON_VERSION --dist-url=https://atom.io/download/electron --directory test; fi
+script:
+  - if [[ -z "$ELECTRON_VERSION" ]]; then node_modules/.bin/tap --gc test/js/*-test.js; fi