Build using Ant under openjdk7

Rather than testing the Ant build under multiple JDKs, I've arbitrarily
decided to test it only under openjdk7.  This keeps us to 4 builds
rather than 6 before the build is reported as green which should be a
bit quicker.

.travis.yml

@@ -4,6 +4,13 @@ jdk:
   - openjdk6
   - oraclejdk7
 sudo: false
+env:
+  global:
+    - BUILD_CMD='make'
+matrix:
+  include:
+    - jdk: openjdk7
+      env: BUILD_CMD='ant -f build.xml'
 cache:
   directories:
   - "${HOME}/dependencies"
@@ -13,7 +20,7 @@ before_install:
   - "sh -e /etc/init.d/xvfb start"
   - "source install_dependencies.sh"
 install:
-  - "make"
+  - "$BUILD_CMD"
   - "cd test"
   - "ln -s $(pwd)/../etc/log4j.properties ."
 script: ant -f build-test.xml test -DEMBOSS_ROOT=$EMBOSS_ROOT | tee test.log && [ -z "$(grep 'Failures:\s[^0]\|Errors:\s[^0]' test.log)" ]