Try to find opt-dir for Heroku buildpack (#28)

On Heroku when building the extension, the taglib files are not
available via an absolute path. Instead, they are in a path like this:

  /tmp/build_0d73acfd-3d71-471a-ad6d-bbd02f5d55d8/vendor/taglib/lib/libtag.so

There are however multiple environment variables, one of them:

  GEM_HOME=/tmp/build_0d73acfd-3d71-471a-ad6d-bbd02f5d55d8/vendor/bundle/ruby/2.0.0

From this, we can extract the vendor path and append taglib to get the
opt dir we need.

It's not very clean, but it feels a bit better than using relative paths
using "..".

Also, it's only tested with the following buildpack, which downloads the
taglib binaries from somewhere (ugh):

https://github.com/mrrad/heroku-buildpack-taglib