Fix ./gradlew eclipse

Running ./gradlew eclipse was failing due to ide.gradle assuming that
SourceFolder's will always have an output. Apparently, this isn't
the case.

This commit makes ide.gradle a little more defensive by using Groovy's
safe navigation to cope with a null output on a SourceFolder.
Merge pull request #202 from wilkinsona/fix-gradlew-eclipse

Fix ./gradlew eclipse