diff --git a/.travis.yml b/.travis.yml
index ae637b2..dafbfcc 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -21,4 +21,4 @@ install:
   - pip install -r requirements.txt
 
 script:
-  - PYTHONPATH=. py.test
+  - PYTHONPATH=. MPLBACKEND="agg" py.test
diff --git a/peartree/paths.py b/peartree/paths.py
index 3a612ba..e5818d8 100644
--- a/peartree/paths.py
+++ b/peartree/paths.py
@@ -43,11 +43,11 @@ def get_representative_feed(file_loc: str,
     else:
         raise NotImplementedError('Unsupported day type string supplied.')
 
-    log(f'Selected_date: {selected_date}')
-    log(f'Number of trips on that date: {trip_count}')
+    log('Selected_date: {}'.format(selected_date))
+    log('Number of trips on that date: {}'.format(trip_count))
 
     all_service_ids = '\n\t'.join(service_ids_by_date[selected_date])
-    log(f'\nAll related service IDs: \n\t{all_service_ids}')
+    log('\nAll related service IDs: \n\t{}'.format(all_service_ids))
 
     sub = service_ids_by_date[selected_date]
     feed_query = {'trips.txt': {'service_id': sub}}