Remove old minor Emacs versions from Travis

Debian Stretch is on 24.5, so we can drop 24.4.
Ubuntu LTS is on 25.2, so we can drop 25.1.

Keeping 25.2 is probably overkill.  But the next Ubuntu LTS is near.

.travis.yml

@@ -24,9 +24,7 @@ cache:
       - "$HOME/.emacs.d/.cask"
 env:
   matrix:
-    - EMACS_VERSION=24.4
     - EMACS_VERSION=24.5
-    - EMACS_VERSION=25.1
     - EMACS_VERSION=25.2
     - EMACS_VERSION=25.3
     - EMACS_VERSION=snapshot