Swift 4 fixes && custom licenses with JSON file

[kmaschke85]

• Fixed accessibility modifier for LicenseContentFetcher bundle property
• Fixed Swift and project warnings
• Added function setLicensesFromJSONFile to add custom licenses via JSON file

[kmaschke85]

Thanks!

[kmaschke85]

@mattwyskiel any plans in near future to release a new pod with these changes?