Remove taskwarrior 2.4.3-2.4.4 from Travis matrix.

Based on @EdwardBetts's comments in #98, it seems that the underlying
bug was introduced in 2.4.3 and fixed in 2.5.0.

Author: ryneeverett

.travis.yml

@@ -13,8 +13,6 @@ env:
  - TASKWARRIOR=v2.4.0
  - TASKWARRIOR=v2.4.1
  - TASKWARRIOR=v2.4.2
- - TASKWARRIOR=v2.4.3
- - TASKWARRIOR=v2.4.4
  - TASKWARRIOR=v2.5.0
  - TASKWARRIOR=v2.5.1
 before_install: