Use `env` to find bash so tests & build can run on *BSD

Simply converted fixed paths on the shebang line from
'/bin/bash' to '/usr/bin/env bash'.  `yokozuna-ami.sh`
also needed a +x flag that was not already set.

Some of the bash scripts can easily be converted to pure
/bin/sh implementations, but it is probably not needed
due to them being build or test related rather than
used in runtime or operations.

Addresses #74
Merge pull request #77 from basho/gh74-fix-bash-paths

Use `env` to find bash so tests & build can run on *BSD