Concat build source-paths with project source-paths

Just ran in to a bug pertaining to this recent change: https://github.com/noprompt/lein-garden/pull/21/files#diff-dd137de1c12ebb1560fec2bfc1052f09R97. If you rely on any dependencies, they no longer get included in the class-path. Leiningen expands `:source-paths` to include your dependency paths, which it needs to add them to the class path.

It looks like lein-cljsbuild concats in the source-paths specified in each build with the original project's source-paths: https://github.com/emezeske/lein-cljsbuild/blob/master/plugin/src/leiningen/cljsbuild/subproject.clj#L102-L105.

This PR just concats the build source-paths with the original project source-paths so you don't lose dependency paths.