Update watch to version 0.3.0

Homebrew thinks the version of the `watch` command it has is version `3.2.8`:

```sh
$ brew info watch
watch: stable 3.2.8
http://procps.sourceforge.net/
Not installed
https://github.com/mxcl/homebrew/commits/master/Library/Formula/watch.rb
```

This is incorrect, because it’s actually the Procps package there whose version is `3.2.8`. When you install watch, you can see that it’s actually version `0.2.0`:

```sh
$ brew install watch
==> Downloading http://procps.sourceforge.net/procps-3.2.8.tar.gz
######################################################################## 100.0%
==> make watch PKG_LDFLAGS=-Wl
/opt/boxen/homebrew/Cellar/watch/3.2.8: 8 files, 76K, built in 2 seconds

$ watch --version
0.2.0
```

This pull updates watch to an OSX port of version `0.3.0` I found, mainly for the extra convenience of supporting colour output with the new `--color` flag that was added. This also sorts out the version Homebrew sees it as.

```sh
$ git checkout cobyism/update-watch ./Library/Formula/watch.rb

$ brew info watch
watch: stable 0.3.0
https://github.com/whit537/watch
Not installed
https://github.com/mxcl/homebrew/commits/master/Library/Formula/watch.rb

$ brew install watch
==> Downloading https://github.com/whit537/watch/archive/0.3.0.tar.gz
######################################################################## 100.0%
/opt/boxen/homebrew/Cellar/watch/0.3.0: 5 files, 60K, built in 2 seconds

$ watch --version
0.3.0

$ watch --help
Usage: watch [-bcdhnptvx] [--beep] [--color] [--differences[=cumulative]] [--exec] [--help] [--interval=<n>] [--no-title] [--version] <command>
  -b, --beep  			beep if the command has a non-zero exit
  -d, --differences[=cumulative]	highlight changes between updates
		(cumulative means highlighting is cumulative)
  -e, --errexit				exit watch if the command has a non-zero exit
  -h, --help				print a summary of the options
  -n, --interval=<seconds>		seconds to wait between updates
  -p, --precise				precise timing, ignore command run time
  -v, --version				print the version number
  -t, --no-title			turns off showing the header
  -x, --exec				pass command to exec instead of sh
```