Exclude .txt files from plugin lib/ when setting commonClasspath

Edited grails-core/src/main/groovy/org/codehaus/groovy/grails/compiler/GrailsProjectCompiler.groovy via GitHub

See http://grails.1312388.n4.nabble.com/Re-Code-Coverage-running-tests-leads-to-huge-stacktrace-tp3418003p3491065.html