Fix database creation

Author: Nicolas Rodriguez

.travis.yml

@@ -18,8 +18,8 @@ services:
 addons:
   postgresql: '9.6'
 before_script:
-  - mysql -e 'create database ajax_datatables_rails;'
-  - psql -c 'create database ajax_datatables_rails;' -U postgres
+  - sh -c "if [ '$DB_ADAPTER' = 'mysql2' ]; then mysql -e 'create database ajax_datatables_rails;'; fi"
+  - sh -c "if [ '$DB_ADAPTER' = 'postgresql' ]; then psql -c 'create database ajax_datatables_rails;' -U postgres; fi"
 env:
   matrix:
     - DB_ADAPTER=postgresql