Remove support for ancient Debian versions we don't use.
We used to grab NSIS from squeeze even if we were not using squeeze, since we needed at least 2.46. Well now squeeze is the oldest version we support, so just install it via apt like normal.
The name of the Java 1.6 alternative has changed from java-6-openjdk to java-1.6.0-openjdk in a recent security update (sigh). So this unbreaks nightlies.