style: reorder jdk versions in .travis.yml

Reorder the openjdk versions in .travis.yml so that openjdk6 comes before openjdk7.

Author: sehrope

.travis.yml

@@ -8,10 +8,10 @@ script: ant test
 
 matrix:
   include:
-    - jdk: openjdk7
+    - jdk: openjdk6
       addons:
         postgresql: "9.1"
-    - jdk: openjdk6
+    - jdk: openjdk7
       addons:
         postgresql: "9.1"
     - jdk: oraclejdk7
@@ -20,10 +20,10 @@ matrix:
     - jdk: oraclejdk8
       addons:
         postgresql: "9.1"
-    - jdk: openjdk7
+    - jdk: openjdk6
       addons:
         postgresql: "9.2"
-    - jdk: openjdk6
+    - jdk: openjdk7
       addons:
         postgresql: "9.2"
     - jdk: oraclejdk7
@@ -32,10 +32,10 @@ matrix:
     - jdk: oraclejdk8
       addons:
         postgresql: "9.2"
-    - jdk: openjdk7
+    - jdk: openjdk6
       addons:
         postgresql: "9.3"
-    - jdk: openjdk6
+    - jdk: openjdk7
       addons:
         postgresql: "9.3"
     - jdk: oraclejdk7