Backport recent .travis.yml changes (#308)

Backported changes from master:

* b52260d8bf
* 984eef7d6d
* 532519770d
* 91b0e7d0ca

.travis.yml

@@ -6,6 +6,11 @@ group: beta
 # To cache doc-building dependencies.
 cache: pip
 
+branches:
+  only:
+    - master
+    - /^\d\.\d$/
+
 os:
   - linux
   # macOS builds are disabled as the machines are under-provisioned on Travis,
@@ -20,6 +25,7 @@ env:
   - TESTING=cpython
 
 matrix:
+  fast_finish: true
   allow_failures:
     - env:
        - TESTING=coverage
@@ -30,17 +36,10 @@ matrix:
       env:
         - TESTING=docs
       before_script:
-        - |
-            if git diff --name-only $TRAVIS_COMMIT_RANGE | grep -qvE '^Doc/'
-            then
-              echo "Docs weren't updated, stopping build process."
-              exit
-            fi
-            cd Doc
-            make venv PYTHON=python3
+        - cd Doc
+        - make venv
       script:
-        - make html SPHINXBUILD="./venv/bin/python3 -m sphinx" SPHINXOPTS="-q"
-        - make check
+        - make check suspicious html PYTHON="./venv/bin/python" SPHINXBUILD="./venv/bin/python -m sphinx" SPHINXOPTS="-q"
     - os: linux
       language: c
       compiler: clang