Using non-posix in shebang for this script to work with Ubuntu

Ubuntu uses a POSIX shell which is dash and thus fails at
```bash
if [[ -z ${COMMAND:-} ]]; then
```
with:
```bash
./git-build.sh: 39: ./git-build.sh: [[: not found
./git-build.sh: 43: ./git-build.sh: Commit: not found
```
According to http://stackoverflow.com/a/10703454, it is preferable to specify bash in shebang to avoid people changing their /bin/sh link... 'It works on my machine' !