Add node/PostgreSQL matrix to .travis.yml

Add a matrix configuration to .travis.yml to test permutations of node and PostgreSQL.
Node versions tested against are v0.10, v0.11, v0.12 and io.js v1.
PostgreSQL versions tested against are 9.1, 9.2, 9.3, and 9.4.

Author: sehrope

.travis.yml

@@ -1,9 +1,56 @@
 language: node_js
-node_js:
-  - "0.10"
-  - "0.12"
-  - "iojs"
 before_script:
   - node script/create-test-tables.js pg://postgres@127.0.0.1:5432/postgres
 env:
   - PGUSER=postgres PGDATABASE=postgres
+
+matrix:
+  include:
+    - node_js: "0.10"
+      addons:
+        postgresql: "9.1"
+    - node_js: "0.10"
+      addons:
+        postgresql: "9.2"
+    - node_js: "0.10"
+      addons:
+        postgresql: "9.3"
+    - node_js: "0.10"
+      addons:
+        postgresql: "9.4"
+    - node_js: "0.11"
+      addons:
+        postgresql: "9.1"
+    - node_js: "0.11"
+      addons:
+        postgresql: "9.2"
+    - node_js: "0.11"
+      addons:
+        postgresql: "9.3"
+    - node_js: "0.11"
+      addons:
+        postgresql: "9.4"
+    - node_js: "0.12"
+      addons:
+        postgresql: "9.1"
+    - node_js: "0.12"
+      addons:
+        postgresql: "9.2"
+    - node_js: "0.12"
+      addons:
+        postgresql: "9.3"
+    - node_js: "0.12"
+      addons:
+        postgresql: "9.4"
+    - node_js: "iojs-v1"
+      addons:
+        postgresql: "9.1"
+    - node_js: "iojs-v1"
+      addons:
+        postgresql: "9.2"
+    - node_js: "iojs-v1"
+      addons:
+        postgresql: "9.3"
+    - node_js: "iojs-v1"
+      addons:
+        postgresql: "9.4"