chore: configure prerelease pipeline

Configures Travis CI and semantic-release to deploy pre-releases with
the `-alpha.XX` version suffix to NPM whenever commits are pushed to
the `alpha` branch.

Author: gruhn

.releaserc.yml

@@ -0,0 +1,4 @@
+branches:
+  - name: master
+  - name: alpha
+    prerelease: true

.travis.yml

@@ -1,5 +1,5 @@
 language: node_js
-node_js: "8.11"
+node_js: lts/*
 
 install:
   - npm install
@@ -10,9 +10,15 @@ script:
 cache: npm
 
 deploy:
-  provider: script
-  skip_cleanup: true
-  on:
-    branch: master
-  script:
-    - npx semantic-release --branch master
+  - provider: script
+    skip_cleanup: true
+    on:
+      branch: master
+    script:
+      - npx semantic-release
+  - provider: script
+    skip_cleanup: true
+    on:
+      branch: alpha
+    script:
+      - npx semantic-release

package.json

@@ -64,7 +64,7 @@
     "inject-loader": "^3.0.0",
     "lint-staged": "^8.1.4",
     "prettier": "1.16.4",
-    "semantic-release": "^15.9.9",
+    "semantic-release": "^17.0.7",
     "vue-loader": "^10.0.0",
     "vue-template-compiler": "^2.1.6",
     "webpack": "^3.0.0",