Fix for issue https://github.com/mwanji/humpty/issues/6

[dtbullock]

This pull request fixes the problem identified in issue #6 with the solution presented there. Note that not all the tests pass for me after this change, but at least not all of them are failing.

Failed tests:   
  should_concatenate_bundle_with_multiple_assets(co.mewf.humpty.PipelineTest): expected:<... synchronize, write;[
  should_compile_bundle(co.mewf.humpty.PipelineTest): expected:<... synchronize, write;[
  should_process_asset_within_bundle(co.mewf.humpty.Pipeline_SingleAssetTest): expected:<... synchronize, write;[

Tests in error: 
  should_resolve_files_in_subdirectory_of_custom_assetsDir(co.mewf.humpty.spi.resolvers.WebJarResolverTest): java.lang.IllegalArgumentException: sub/asset2.js could not be found. Make sure you've added the corresponding WebJar and please check for typos.
  should_resolve_files_in_default_assetsDir(co.mewf.humpty.spi.resolvers.WebJarResolverTest): java.lang.IllegalArgumentException: asset1.js could not be found. Make sure you've added the corresponding WebJar and please check for typos.
  should_resolve_files_in_custom_assetsDir(co.mewf.humpty.spi.resolvers.WebJarResolverTest): java.lang.IllegalArgumentException: asset1.js could not be found. Make sure you've added the corresponding WebJar and please check for typos.