Add Solidus v2.6 to .travis.yml

Author: John Hawthorn

.travis.yml

@@ -21,6 +21,7 @@ env:
     - 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=master DB=postgres
     - SOLIDUS_BRANCH=v1.1 DB=mysql
     - SOLIDUS_BRANCH=v1.2 DB=mysql
@@ -32,4 +33,5 @@ env:
     - 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=master DB=mysql