Set CEYLON_HOME correctly for bootstrap build

Fixed the problem for new developers, who will get `ceylon.language/build.xml:189: Error running Ceylon compile tool (an exception was thrown, run ant with -v parameter to see the exception)`, because `<ceylon-compile>` cannot find the correct `ceylon` command.
Merge pull request #246 from HenningB/patch-1

Set CEYLON_HOME correctly for bootstrap build