update google closure compiler dependency, fix JSSourceFile -> SourceFile

JSSourceFile was renamed, asset minification with v20141120 fails with
java.lang.ClassNotFoundException: com.google.javascript.jscomp.JSSourceFile

this commit updates the closure compiler dependency, bumps the version to
0.1.5-SNAPSHOT, and renames s/JSSourceFile/SourceFile/g
Merge pull request #2 from blak3mill3r/master

fix for latest Closure Compiler