travis: Build with OracleJDK8 rather than OpenJDK8

The travis documentation was wrong; it's actually "oraclejdk8", not "openjdk" as they implied.

Note that the source won't currently build with jdk8:
```
[ERROR] /home/travis/build/rleigh-dundee/bioformats/components/forks/jai/src/com/sun/media/imageioimpl/plugins/pnm/PNMImageWriter.java:[148,38] GetPropertyAction is internal proprietary API and may be removed in a future release
[ERROR] /home/travis/build/rleigh-dundee/bioformats/components/forks/jai/src/com/sun/media/imageioimpl/plugins/pnm/PNMImageReader.java:[136,38] GetPropertyAction is internal proprietary API and may be removed in a future release
```

Is this something which can be fixed easily?