[pitivi] Fix deprecated arguments to Matplotlib function.

[Thibault Saunier <tsaunier src gnome org>]

commit 7c53a2438775a8d7aeef3e5fd650537c073f6f63
Author: Elliott Sales de Andrade <quantum analyst gmail com>
Date:   Sun Jun 17 02:28:42 2018 -0400

    Fix deprecated arguments to Matplotlib function.

 pitivi/timeline/elements.py | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)
---
diff --git a/pitivi/timeline/elements.py b/pitivi/timeline/elements.py
index dbc546ab..6b6bb399 100644
--- a/pitivi/timeline/elements.py
+++ b/pitivi/timeline/elements.py
@@ -123,10 +123,10 @@ class KeyframeCurve(FigureCanvas, Loggable):
 
         self._ax.tick_params(axis='both',
                              which='both',
-                             bottom='off',
-                             top='off',
-                             right='off',
-                             left='off')
+                             bottom=False,
+                             top=False,
+                             right=False,
+                             left=False)
 
         # This seems to also be necessary for transparency ..
         figure.patch.set_visible(False)