Simplify making the docs on travis with make -C

[skip appveyor]

.travis.yml

@@ -22,7 +22,7 @@ install:
 script:
     - if [[ $TRAVIS_PYTHON_VERSION != pypy* ]]; then bin/coverage run bin/coverage-test -v1j99; fi
     - if [[ $TRAVIS_PYTHON_VERSION == pypy* ]]; then bin/test -v1j99; fi
-    - if [[ $TRAVIS_PYTHON_VERSION != pypy3* ]]; then pushd doc; make html; popd; fi
+    - if [[ $TRAVIS_PYTHON_VERSION != pypy3* ]]; then make -C doc html; fi
     - if [[ $TRAVIS_PYTHON_VERSION != pypy* ]]; then pip install coveralls; fi # install early enough to get into the cache
 after_success:
   - if [[ $TRAVIS_PYTHON_VERSION != pypy* ]]; then bin/coverage combine; fi