script: build-integration-branch: avoid Unicode error

script: build-integration-branch: avoid Unicode error

When run on PRs that have non-ASCII characters in their titles, the script
fails like this when run in a non UTF-8 environment:

UnicodeEncodeError: 'latin-1' codec can't encode character u'\u2026' in
position 651: ordinal not in range(256)

Since failing like that is not useful, avoid the error by forcing ASCII
encoding and filtering out any non-ASCII characters.

Fixes: http://tracker.ceph.com/issues/24003
Signed-off-by: Nathan Cutler <ncutler@suse.com>