Verify that pull requests are based on develop

.travis.yml

@@ -1,3 +1,4 @@
 language: cpp
 script:
 - python travis/lint.py
+- python travis/pr-check-base.py

travis/pr-check-base.py

@@ -0,0 +1,29 @@
+import json, os, sys
+if sys.version.startswith('2'):
+    from urllib2 import urlopen, HTTPError
+else:
+    from urllib.request import urlopen
+    from urllib.error import HTTPError
+try:
+    pr_id = int(os.environ.get('TRAVIS_PULL_REQUEST', 'false'))
+except ValueError:
+    print('Not a pull request')
+    sys.exit(0)
+print('Pull request %i' % pr_id)
+res = {}
+try:
+    res = json.loads(urlopen('https://api.github.com/repos/dfhack/dfhack/pulls/%i' % pr_id).read().decode('utf-8'))
+except ValueError:
+    pass
+except HTTPError:
+    print('Failed to retrieve PR information from API')
+    sys.exit(2)
+if 'base' not in res or 'ref' not in res['base']:
+    print('Invalid JSON returned from API')
+    sys.exit(2)
+if res['base']['ref'] != 'develop':
+    print('Not based on develop branch')
+    sys.exit(1)
+else:
+    print('Ok')
+    sys.exit(0)