Fixed the .ruby-version file to prevent annoying warning

Using rbenv, it provides a warning when the version number in the '.ruby-version' file contains the full ruby string. It needs to just have the version number. An example of the warning is below:

```
420-1572-MAN:metasploit-framework mandreko$ bundle install
warning: ignoring extraneous `ruby-' prefix in version `ruby-1.9.3-p448'
         (set by /Users/mandreko/Development/metasploit-framework/.ruby-version)
Using rake (10.1.0)
Using i18n (0.6.5)
Using multi_json (1.0.4)
Using activesupport (3.2.14)
Using builder (3.0.4)
Using activemodel (3.2.14)
Using arel (3.0.2)
Using tzinfo (0.3.37)
Using activerecord (3.2.14)
Using database_cleaner (1.1.1)
Using diff-lcs (1.2.4)
Using factory_girl (4.2.0)
Using json (1.8.0)
Using pg (0.16.0)
Using metasploit_data_models (0.16.6)
Using mini_portile (0.5.1)
Using msgpack (0.5.5)
Using network_interface (0.0.1)
Using nokogiri (1.6.0)
Using packetfu (1.1.9)
Using pcaprub (0.11.3)
Using redcarpet (3.0.0)
Using robots (0.10.1)
Using rspec-core (2.14.5)
Using rspec-expectations (2.14.2)
Using rspec-mocks (2.14.3)
Using rspec (2.14.1)
Using shoulda-matchers (2.3.0)
Using simplecov-html (0.5.3)
Using simplecov (0.5.4)
Using timecop (0.6.3)
Using yard (0.8.7)
Using bundler (1.3.5)
Your bundle is complete!
Use `bundle show [gemname]` to see where a bundled gem is installed.
```

Note the warning. Correcting the version number removes this annoying warning.