run install.sh with `sh` rather than `bash`...

...since bash-isms seem to be removed in latest install.sh

Latest https://www.opscode.com/chef/install.sh from 25/02/2013 says:
```
#!/bin/sh
# WARNING: REQUIRES /bin/sh
#
# - must run on /bin/sh on solaris 9
# - must run on /bin/sh on AIX 6.x
# - if you think you are a bash wizard, you probably do not understand
#   this programming language.  do not touch.
# - if you are under 40, get peer review from your elders.
#
...
```