Update .travis.yml with explicit attrs upgrade

python3.6 on travis comes with attrs 17.3.0 already installed. Since this creates a conflict with databroker -> jsonschema -> attrs (17.4.0), immediately upgrade the attrs package to at least 17.4.0 before installing anything else.

.travis.yml

@@ -14,8 +14,8 @@ addons:
     packages:
     - qtbase5-dev
 install:
+- pip install --upgrade attrs>=17.4.0 # travis python3.6 has attrs 17.3.0 installed
 - pip install cx_freeze==6.0b1
-- pip install databroker # avoid attrs version conflict
 - pip install coveralls pytest
 - pip install git+https://github.com/lbl-camera/Xi-cam.core.git
 - pip install git+https://github.com/lbl-camera/Xi-cam.plugins.git