Use Darcs 2.4.4 on Travis #1142

We are not yet compatible to Darcs >= 2.5, so we use 2.4.4 until we
are compatible with newer versions.

The GCONV_PATH environment variable is required for the precompiled
Darcs binary tio find gconv and to not die with

darcs: mkTextEncoding: invalid argument (Invalid argument)

See also http://web.archive.org/web/20100821192352/http://blog.malde.org/index.php/2010/05/22/snagged/