build docker image first

.travis.yml

@@ -3,6 +3,9 @@ sudo: required
 services:
   - docker
 
+before_script:
+- docker build -t 'developmentseed/landsat-util:travis' .
+
 script:
 - docker run --rm -it -v "$(pwd)":/test developmentseed/landsat-util:travis nosetests
 

README.rst

@@ -32,7 +32,7 @@ To run the documentation locally::
 Travis Tests
 ++++++++++++
 
-To speed up testing on travis, we use a docker image. If you add new packages, to ensure that tests will pass on Travis, make sure to rebuild the docker image and push it to DockerHub with `travis` tag.
+To speed up testing on travis, we use a docker image.
 
 To test with docker image locally run: