FIX: replace env var with python version

Author: chiahaoliu

.travis.yml

@@ -10,7 +10,6 @@ python:
   - 2.7
   - 3.5
   - 3.6
-    env: BUILD_DOCS=1
 
 before_install:
   - wget https://repo.continuum.io/miniconda/Miniconda3-latest-Linux-x86_64.sh -O miniconda.sh
@@ -39,7 +38,7 @@ script:
   - coverage report -m
   - codecov
   - |
-    if [ $BUILD_DOCS ]; then
+    if [ $TRAVIS_PYTHON_VERSION = "3.6" ]; then
       conda install --yes --file requirements/docs.txt
       cd docs
       make html