FIX: typo in environment variable

Author: chiahaoliu

.travis.yml

@@ -10,7 +10,7 @@ python:
   - 2.7
   - 3.5
   - 3.6
-    env: BULD_DOCS=1
+    env: BUILD_DOCS=1
 
 before_install:
   - wget https://repo.continuum.io/miniconda/Miniconda3-latest-Linux-x86_64.sh -O miniconda.sh