Fixes version string for gem

:information_desk_person: These changes correct the gem version used and remove the `Gemfile.lock` file.
```
$ bundle
There was a ArgumentError while loading rspec_api_documentation.gemspec: 
Malformed version number string 2.0.1-bc from
 ~/src/rspec_api_documentation/rspec_api_documentation.gemspec:6:in `block in <main>'
```

Taken from ['Clarifying the roles of the .gemspec and Gemfile'](http://yehudakatz.com/2010/12/16/clarifying-the-roles-of-the-gemspec-and-gemfile/):
> Do not check your Gemfile.lock into version control, since it enforces precision that does not exist in the gem command, which is used to install gems in practice. Even if the precision could be enforced, you wouldn’t want it, since it would prevent people from using your library with versions of its dependencies that are different from the ones you used to develop the gem.