duplicity: Fix installation with distutils.cfg

The Homebrew Python installations place a `distutils.cfg` file that can override
arguments passed on the command line. Specifically, our `distutils.cfg` sets a
value for `install-lib`, so `--install-lib` needs to be passed to `setup.py` to
override this default. Passing `--install-purelib` and `--install-platlib` is
not sufficient.

Formula/duplicity.rb

@@ -13,8 +13,7 @@ class Duplicity < Formula
     # Install mostly into libexec
     system "python", "setup.py", "install",
                      "--prefix=#{prefix}",
-                     "--install-purelib=#{libexec}",
-                     "--install-platlib=#{libexec}",
+                     "--install-lib=#{libexec}",
                      "--install-scripts=#{bin}"
 
     # Shift files around to avoid needing a PYTHONPATH