Treat pushing a second mouse button during a rectangle zoom as "cancel" rather than putting us in an unresolved state.
Fix so that zooming continues to work even after a box was canceled
Merge pull request #1944 from mdboom/cancel-zoom

ValueError exception in drag_zoom (tk backend)
Remove trailing backslash as pointed out by @dmcdougall
Don't use the font cache on TRAVIS
Merge pull request #2132 from mdboom/font_cache_fix

Attempt to fix Travis tests by not using font cache there.
Tagging v1.3.0rc3 relase