Publish branches as npm tags

Although this does not yet add the ability to publish stable
releases to npm from CI, this does publish every newly pushed
branch to npm using that branch's name as the tag. This will allow
us to test whether a package still works properly after it's
published and built.

Author: Vinnl

.travis.yml

@@ -5,3 +5,15 @@ script:
   - npm test
   # Make sure it still builds successfully - the build isn't actually used yet:
   - npm run build
+before_deploy:
+  - node ./travis/bumpversion.js
+deploy:
+  provider: npm
+  # Do not throw away the updated package.json we generated in `before_deploy`:
+  skip_cleanup: true
+  email: "$NPM_EMAIL"
+  api_key: "$NPM_TOKEN"
+  # Note: do not deploy on pull request, because $TRAVIS_BRANCH will be the target branch.
+  tag: "$TRAVIS_BRANCH"
+  on:
+    all_branches: true

travis/bumpversion.js

@@ -0,0 +1,19 @@
+/**
+ * npm does not allow you to publish a package with the same version multiple times.
+ * Thus, to publish prerelease versions tagged to the branch they're built from,
+ * we need to generate unique version numbers that are also higher than versions already published.
+ * To achieve this, we append `build<build_number>` to the version number.
+ * The actual release version can eventually be published without the suffix.
+ */
+
+if (!process.env.TRAVIS_BUILD_NUMBER || process.env.TRAVIS_BUILD_NUMBER.length === 0) {
+  console.error('Could not read the build number to bump the package version - aborting publish.')
+  process.exit(1)
+}
+
+const fs = require('fs')
+const path = require('path')
+
+const packageJson = require('../package.json')
+packageJson.version = `${packageJson.version}build${process.env.TRAVIS_BUILD_NUMBER}`
+fs.writeFileSync(path.resolve(__dirname, '../package.json'), JSON.stringify(packageJson))