Install MongoDB FDW on Travis CI

I needed to install the MongoDB FDW for Postgresql on Travis CI and this worked for me. Assumes version PG v9.3.

# in travis.yml
before_script:
  - script/install_fdw

# script/install_fdw
#!/bin/sh

sudo apt-get install postgresql-server-dev-9.3
git clone https://github.com/citusdata/mongo_fdw
cd mongo_fdw/
PATH=/usr/lib/postgresql/9.3/bin/:$PATH make
sudo PATH=/usr/lib/postgresql/9.3/bin/:$PATH make install