`install_dmd_package` does not work for dmd-2.071.0

[tom-tan]

DMD 2.071 was released and dlang.org provides a zip file for each operating system.

The problem is that the links for FreeBSD have different name convention from other OSes. i.e. it contains architecture information: 32 or 64 that is different from $arch variable (introduced in #8).

It seems that we cannot use only one download link for all platforms and architectures...

Here are the download links for dmd 2.070.2 and 2.071.0 for some platforms.

For dmd 2.070.2:

• For all platforms: http://downloads.dlang.org/releases/2.x/2.070.2/dmd.2.070.2.zip

For dmd 2.071.0:

• For Linux: http://downloads.dlang.org/releases/2.x/2.071.0/dmd.2.071.0.linux.zip
• For FreeBSD (32bit): http://downloads.dlang.org/releases/2.x/2.071.0/dmd.2.071.0.freebsd-32.zip
• etc

We can use $os variable (introduced in Issue #8) to handle OS names such as linux, osx and others.