fix Grunt on unix

fix file case to avoid this error when building:

    mathieu on ElasticUI:master ⚡ grunt
    Running "typescript:dist" (typescript) task
    >> /data/users/mathieu/prelinker-debug/app/bower_components/ElasticUI/_references.ts(40,1):
    >> error TS5007: Cannot resolve referenced file: 'src/widgets/directives/checklistdirective.ts'.
    >> /data/users/mathieu/prelinker-debug/app/bower_components/ElasticUI/_references.ts(41,1):
    >> error TS5007: Cannot resolve referenced file: 'src/widgets/directives/simplepagingdirective.ts'.
    >> /data/users/mathieu/prelinker-debug/app/bower_components/ElasticUI/_references.ts(42,1):
    >> error TS5007: Cannot resolve referenced file: 'src/widgets/directives/singleselectdirective.ts'.