Fixed typo

.travis.yml

@@ -8,7 +8,7 @@ services:
   - docker
 
 before_install:
-  - docker built -t opendronemap/node-opendronemap .
+  - docker build -t opendronemap/node-opendronemap .
   - docker run opendronemap/node-opendronemap /bin/sh -c "cd /var/www; node index.js --powercycle"
 
 script: node index.js --powercycle