TST : update the version of minicoda

Update the version of minicoda used to boot-strap the VM
TST : give miniconda a prefix

new versions of miniconda3 install (by default) to ~/miniconda3
which means it doesn't get added to the path by the next command.

This forces it to install to ~/miniconda, independent of version.
Merge pull request #53 from tacaswell/miniconda_update

TST : update the version of minicoda