add Danger step to Travis + Danger configuration

.travis.yml

@@ -22,7 +22,9 @@ addons:
 
 install:
   - pip install tox-travis
+  - gem install danger --version '~> 5.0'
 
 script:
   - tox
   - tox -e patched_loops
+  - danger

Dangerfile

@@ -0,0 +1,7 @@
+# Lovingly stolen from https://github.com/realm/jazzy/blob/master/Dangerfile
+
+has_app_changes = !git.modified_files.grep(/cassandra/).empty?
+if !git.modified_files.include?("CHANGELOG.rst") && has_app_changes
+  fail "You've modified application code. " +
+    "Unless this is strictly a refactoring, please include a CHANGELOG entry."
+end