GROOVY-6490

http://jira.codehaus.org/browse/GROOVY-6490 excessive reload of dependencies in GroovyScriptEngine. Solve storing the last-modified attribute from the source file in the scriptCache.
Merge pull request #297 from alessiostalla/patch-1

GROOVY-6490: correct last modified time
GROOVY-6490: avoid a lastmodified time in the future, force removal of up-to-date dependencies