Fix flake8_diff.sh when Travis runs in a fork

The grep was not matching anything so the script exited with a 1 exit code.

Author: lesteve

build_tools/travis/flake8_diff.sh

@@ -24,7 +24,7 @@ echo "Remotes:"
 git remote --verbose
 
 # Find the remote with the project name (upstream in most cases)
-REMOTE=$(git remote -v | grep $PROJECT | cut -f1 | head -1)
+REMOTE=$(git remote -v | grep $PROJECT | cut -f1 | head -1 || echo '')
 
 # Add a temporary remote if needed. For example this is necessary when
 # Travis is configured to run in a fork. In this case 'origin' is the