Add travis check for the base branch (only permit develop for code changes)

.travis.yml

@@ -2,6 +2,7 @@ language: node_js
 node_js: lts/*
 
 script:
+- bash .travis/prevent-base-master.sh
 - bash .travis/omit-dist.sh
 - npm run lint
 - npm run build

.travis/prevent-base-master.sh

@@ -0,0 +1,5 @@
+#!/bin/bash
+if [ "$TRAVIS_PULL_REQUEST" != "false" ] && [ $TRAVIS_BRANCH == "master" ] && $(git diff --name-only $TRAVIS_COMMIT_RANGE | grep -q "^src/"); then
+	echo 'The base branch for pull requests must be "develop"' >&2
+	exit 1
+fi