Swift 5.1 is now the minimum supported version

CI update and remove swift 4.x support (#82)

* Remove Swift 4.x support; Update TravisCI

* Update SPM syntax and minimum version dependency on swift-log

1.9.0

• HeliumLogger may now be used as a swift-log backend (#73)

1.8.1

• Support Swift 5

1.8.0

• Add support for building with Swift 5 (#70)
• Drop support for Swift 3.

Note: if you require Swift 3 support, please continue to use the 1.7 release of HeliumLogger.

1.7.3

• Fix manifest parse error on Swift 4.2

1.7.2

• Ensure that Swift 4 Package.swift is always used on Swift 4

1.7.1

What's New

• Support Swift 4

1.6.1

Conditional Swift-3.1 compatibility

0.5.0: Adding formatting options.

• The format is now able to be set.
• Filename no longer contains the absolute path

0.4.2

Update Package.swift