Upgrade casadi to have the fatrop solver

fatrop knows how to use sparsity correctly, once we set it up.

Change-Id: Ib25be3fecd89dd4a2f670f7120a383e6dce34811
Signed-off-by: Austin Schuh <austin.linux@gmail.com>
diff --git a/tools/python/requirements.txt b/tools/python/requirements.txt
index 086b138..48b58ff 100644
--- a/tools/python/requirements.txt
+++ b/tools/python/requirements.txt
@@ -24,4 +24,4 @@
 bokeh
 tabulate
 
-casadi
+casadi>=3.6.6