Quick fix for ScalaLanguageLevel crashing with External build compiler

ScalaLanguageLevel.getLanguageLevel(element) would crash if
element.getContainingFile.getVirtualFile was null. This would pretty
much kill any Completion/Highlighting processes that touched it, as
in this bug report against Completion:

http://youtrack.jetbrains.com/issue/SCL-5108

I think getVirtualFile() is supposed to return null iff the file
exists only in memory, however this was occurring for me with Scala
0.7.62/IU-123.94 on concrete *.scala source files.

I previously thought that this was occurring only when 'Use External
build' was enabled, but in fact the problem with getVirtualFile()
returning null seems independent of that.

http://blog.jetbrains.com/scala/2012/12/28/a-new-way-to-compile/
Merge pull request #21 from rtyley/master

Fix for ScalaLanguageLevel crashing with External build compiler