updating classpath during installation might fail for an unknown reason.
Therefore, make the process more robust by wrapping up in try-catch block
and ignoring a potential error. This can have effect only when
the plugin installation is part of a grails tasks that requires
code compilation and could be work arounded by invoking
grails refresh-dependencies first
fixed resolving validation-api source jar via grails 2 dependency manager:
instead of specifying ivy configuration at resolving process,
use m:classifier extra attribute at ModuleRevisionId
Merge pull request #17 from dawsonsystems/source-dependencies-fix

Source dependencies + Install fix