add the postgis 2.3 due to postgres 9.6 dependency

.travis.yml

@@ -3,7 +3,8 @@ python:
  - "3.5"
  - "3.5-dev" # 3.5 development branch
  - "3.6"
-install: "pip install -r requirements/dev.txt"
+install: 
+ - "pip install -r requirements/dev.txt"
 branches:
  only:
  - /^develop.*$/ #only the develop branches will be tested
@@ -14,6 +15,9 @@ before_script:
  - psql -c 'CREATE EXTENSION postgis;' -U postgres -d travis_test
 addons:
  postgresql: "9.6"
+ apt:
+ packages:
+ - postgresql-9.6-postgis-2.3
 # command to run tests
 script: pytest