Uses GNU and OSX compatible mktemp

[jjzieve]

I realized we're assuming GTCtoVCF will be run on Linux systems, especially for production usage. For example, when the README points to the Linux Miniconda install script. But it seems to work fine on Darwin with a couple tweaks to the download_reference.sh script. And users may want to run locally on a Mac if they're just testing it out.

• Main tweak is to deal with differences between GNU mktemp and OSX mktemp. This SO post explains it: https://unix.stackexchange.com/questions/30091/fix-or-alternative-for-mktemp-in-os-x
• The other tweak is just to help with ambiguous errors. This SO post explains that: https://stackoverflow.com/questions/2462385/getting-an-ambiguous-redirect-error
• Updated link to build 38 (pointed out in #54)

[jjzieve]

Also forgot I had to run brew install coreutils as well