Add dependency version checking

[3flex]

A hopefully improved version of #9. Closes #24.

Changes compared to #9:

• Uses the new semantic_puppet gem for parsing and validation of the version_requirement. This means validation code does not need to be maintained in this gem, and works around #10. If Puppet decides to change their validation rules, no action would be required here.
• Addresses the issues raised in #11, and tests for them. The only exception is that an empty version_requirement field (i.e. "version_requirement": "") is treated as an error by semantic_puppet, so this continues to throw an error. But "dependencies": [] will not throw an error, and { "name": "puppetlabs/stdlib" } will be treated as an open ended dependency.
• Several additional tests

Issues with this PR:

• Rubocop issues failing the build. If #29 is merged it will solve the build errors.
• semantic_puppet may be too strict. This PR currently causes error state when semantic_puppet can't/won't parse the version dependency. This is probably the right behaviour, but might cause some short-term pain.
• semantic_puppet code is subject to change. Their README states

Note that this is a 0 release version, and things can change. Expect that the version and version range code to stay relatively stable, but the module dependency code is expected to change.

If this holds true, there should be low risk of this code breaking due to changes in that gem, as it relies only on the version range code which should stay "relatively stable".

[bastelfreak]

wow, this looks really good. Always cool to see such a detailed PR description, thanks for the work! I've to talk a look tomorrow when I'm really awake again. Please ping any of us in #voxpupuli on freenode if we don't do any progress here in the next days (means: I will probably forget that I want to review this in a few hours :( )

[bastelfreak]

also we should try to modulesync this, even if it is a gem

[3flex]

I've corrected my "note to self" comments and rebased, so this is good for a review.

[electrical]

Looks good to me!

[3flex]

@domcleal rubocop's happy now, and I've versioned the semantic_puppet gem to >= 0.1.2 < 1.0.0

[domcleal]

Much better, one style comment inline about the global-ish variable. Unfortunately I'm not a committer, but otherwise it works and looks good to me too.

[3flex]

Thanks for the feedback! I hope this is OK now :)