install-prestoadmin script does not work when run outside prestoadmin directory

[mattsfuller]

The presto-admin installation script looks for directories relative to the current path and not relative to the script itself. Therefore, when you run it from outside of the prestoadmin directory

1. it can't find the third party directory
2. it can't find wheel

If it checked explicitly within the directory the script was in, the script could be run from anywhere.