Remove versions past EOL from .travis.yml

Refs solidusio/solidus#2866 (comment)

There is no reason to invest resources in testing extensions against Solidus versions that have passed EOL.

[Solidus Version Maintenance/EOL policy](https://solidus.io/blog/2018/01/04/maintenance-eol-policy.html)

.travis.yml

@@ -4,15 +4,13 @@ rvm:
   - 2.3.1
 env:
   matrix:
-    - SOLIDUS_BRANCH=v2.1 DB=postgres
     - SOLIDUS_BRANCH=v2.2 DB=postgres
     - 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=master DB=postgres
-    - SOLIDUS_BRANCH=v2.1 DB=mysql
     - SOLIDUS_BRANCH=v2.2 DB=mysql
     - SOLIDUS_BRANCH=v2.3 DB=mysql
     - SOLIDUS_BRANCH=v2.4 DB=mysql