Assimp3 install and dash compatibility

I started fixing the *assimp2* uninstall and *assimp3* install steps and ended up removing all bashisms from the allocore's install dependencies script.

A few points:
1. The fixes to the *mingw* stanza need to be tested by someone with that platform.
2. I need to go over the other install dependencies scripts... and all *"sh"* scripts for that matter.
3. We ensure that *assimp3* is used because compilation completes, but is wrong if there is a *assimp2* and *assimp3* mismatch.

Canonical's [guide for porting from bash to dash](https://wiki.ubuntu.com/DashAsBinSh) was useful.