Fix gemspec, file man/guard.2 does not exist

Fixes the following warning when installing latest guard's master:

```
guard at path/to/guard did not have a valid gemspec.
This prevents bundler from installing bins or native extensions, but
that may not affect its functionality.
The validation message from Rubygems was:
  ["man/guard.2"] are not files
```
Merge pull request #620 from deivid-rodriguez/fix_gemspec

Fix gemspec, file man/guard.2 does not exist