Add ":file" to the list of supported sources

[legal90]

Description

That is a continuation of #795.
In order to use file fetcher in source DSL method of software, we need to add it to the list of valid arguments. Otherwise, the following error will occur:

Invalid value for `source'. Expected source to only include valid keys. Invalid keys: [:file]!

Use case:

# config/software/my_software.rb
# <...>

version("local_file") do
  source file: "../../my_dir/artifact.zip"
end

# <...>

[scotthain]

Looks good, thanks!

[legal90]

Thank you for the review!
I've added tests and TravisCI build passes fine. However, the Appveyor build fails, but it is not related to this changeset.

[scotthain]

Hi @legal90 I believe we've resolved the appveyor issues in master - could you rebase this PR? Thanks!

[legal90]

@scotthain Sure! I've rebased the branch.

[legal90]

Just did it once again.

[robbkidd]

[scotthain]

Thanks @legal90 - sorry it took so long to get things cleared up!