Remove direct pg dependency

refs #1333
- Adjust Travis build to install pg
- Remove pg from package.json

.travis.yml

@@ -19,7 +19,7 @@ before_install:
   - export PATH=$PATH:`pwd`/bin
   - cd -
   - if [ $DB == "mysql" ]; then mysql -e 'create database ghost_travis'; fi
-  - if [ $DB == "pg" ]; then psql -c 'create database ghost_travis;' -U postgres; fi
+  - if [ $DB == "pg" ]; then npm install pg; psql -c 'create database ghost_travis;' -U postgres; fi
 before_script:
   - phantomjs --version
   - casperjs --version

package.json

@@ -55,8 +55,7 @@
         "when": "2.5.1"
     },
     "optionalDependencies": {
-        "mysql": "2.0.0-alpha9",
-        "pg": "~2.6.2"
+        "mysql": "2.0.0-alpha9"
     },
     "devDependencies": {
         "blanket": "~1.1.5",