Installing from the cache skips files a gem's build generates besides compiled libraries #36

Related to #33.

When a gem's build produces files other than compiled libraries, such as generated Ruby files, executables or data files, installing it from the cache brings back only the compiled libraries. If such a gem also has a compiled library, the install reports success and the gem fails later when it looks for a file that was never restored. No cached gem fails this way today, but sass-embedded, whose build produces an executable and no library, downloads its cached copy on every install and then builds locally anyway.

A gem installed from the cache should contain every file that building it locally would produce.