Test with multiple TypeScript minor versions in CI

Fixes #216

Author: nickserv

.travis.yml

@@ -1,2 +1,10 @@
 language: node_js
 node_js: node
+env:
+  - TYPESCRIPT_VERSION=3.6
+  - TYPESCRIPT_VERSION=3.5
+  - TYPESCRIPT_VERSION=3.4
+  - TYPESCRIPT_VERSION=3.3
+install: npm ci --ignore-scripts
+after_install: npm install typescript@$TYPESCRIPT_VERSION
+script: npm run prepare