Expand Travis test matrix

Author: piyushk

.travis.yml

@@ -16,13 +16,16 @@ env:
   - SPHINX_VERSION=1.8.*
   - SPHINX_VERSION=2.0.*
   - SPHINX_VERSION=2.1.*
+  - SPHINX_VERSION=2.2.*
   - SPHINX_VERSION=""
 matrix:
   exclude:
     - python: "2.7"
       env: SPHINX_VERSION=2.0.*
     - python: "2.7"
       env: SPHINX_VERSION=2.1.*
+    - python: "2.7"
+      env: SPHINX_VERSION=2.2.*
     - python: "3.7"
       env: SPHINX_VERSION=1.3.*
     - python: "3.7"