install tmux version >= 1.8

the copy functions for tmux on mac osx depend on the `copy-pipe` function to be available. That function was implemented in tmux version 1.8. This code ensures that tmux version >= 1.8 is installed. The previous code only checked if any tmux version was installed.
made tmux 1.8 check more robust

Now using `brew list tmux --versions`
```
       ls, list [--unbrewed] [--versions] [--pinned] [formulae]
              Without any arguments, list all installed formulae.

              If formulae are given, list the installed files for formulae. Combined with --verbose, recursively list the contents of all sub-
              directories in each formula's keg.

              If --versions is passed, show the version number for installed formulae, or only the specified formulae if formulae are given.

```
returns `tmux 1.9a` on my machine. Grab second part of this string. Creates a Gem::Version object and compares it that way.
Merge pull request #144 from morenoh149/patch-3

install tmux version >= 1.8