Add emacs 25.3, emacs-26 and master to the Travis matrix

Remove emacs 25.2.

.travis.yml

@@ -6,12 +6,14 @@ env:
   global:
     - CURL="curl -fsSkL --retry 9 --retry-delay 9"
   matrix:
+  # https://github.com/npostavs/emacs-travis/releases
   - EMACS_VERSION=24.4
   - EMACS_VERSION=24.5
   - EMACS_VERSION=25.1
-  - EMACS_VERSION=25.2
-  - EMACS_VERSION=25 # emacs-25 branch, built daily
-  # - EMACS_VERSION=master # master branch, built daily
+  - EMACS_VERSION=25.3
+  - EMACS_VERSION=25 # emacs-25 branch
+  - EMACS_VERSION=26 # emacs-26 branch
+  - EMACS_VERSION=master # master branch
 install:
   - $CURL -O https://github.com/npostavs/emacs-travis/releases/download/bins/emacs-bin-${EMACS_VERSION}.tar.gz
   - tar xf emacs-bin-${EMACS_VERSION}.tar.gz -C /