Update Travis build matrix

It introduces the following changes:

* Add Solidus v2.8
* Remove Solidus v2.3

Author: aitbw

.travis.yml

@@ -9,15 +9,15 @@ rvm:
   - 2.5
 env:
   matrix:
-    - SOLIDUS_BRANCH=v2.3 DB=postgres
     - SOLIDUS_BRANCH=v2.4 DB=postgres
     - SOLIDUS_BRANCH=v2.5 DB=postgres
     - SOLIDUS_BRANCH=v2.6 DB=postgres
     - SOLIDUS_BRANCH=v2.7 DB=postgres
+    - SOLIDUS_BRANCH=v2.8 DB=postgres
     - SOLIDUS_BRANCH=master DB=postgres
-    - SOLIDUS_BRANCH=v2.3 DB=mysql
     - SOLIDUS_BRANCH=v2.4 DB=mysql
     - SOLIDUS_BRANCH=v2.5 DB=mysql
     - SOLIDUS_BRANCH=v2.6 DB=mysql
     - SOLIDUS_BRANCH=v2.7 DB=mysql
+    - SOLIDUS_BRANCH=v2.8 DB=mysql
     - SOLIDUS_BRANCH=master DB=mysql