fixing circleci lowercase issue

circleci had the following issue:
```
Build-agent version 0.0.4869-fac853b (2018-04-17T20:59:55+0000)
Starting container SpectroscoPyx/documentation-builder:latest
  image cache not found on this host, downloading SpectroscoPyx/documentation-builder:latest
invalid reference format: repository name must be lowercase
```

.circleci/config.yml

@@ -19,7 +19,7 @@ version: 2
 jobs:
   test-html:
     docker:
-      - image: SpectroscoPyx/documentation-builder:latest
+      - image: spectroscopyx/documentation-builder:latest
     steps:
       - checkout
       - run: *deps-install