Upgrade to use swift 4.2

[mtjoe]

This PR contains the following changes:

• Upgrade MMPixelSDK to use Swift 4.2
• Upgrade MMPixelExampleApp to use Swift 4.2
• Fix Swift errors
• Fix failing tests

Upon upgrading a Swift 4.2, there was a few errors:

1. In MMPixelSDK/MMPixelConfig.swift, line 35 & 37:

• Overlapping accesses to 'name', but modification requires exclusive access; consider copying to a local variable
• Fixed by changing the 5th argument of sysctl to nil. This argument (newval) is not required to get the hardware name of the device.

2. In MMPixelSDK/MMPixelSDK.swift, line 107:

• Initializer for conditional binding must have Optional type, not 'UUID'
• Fixed by removing the conditional binding.

3. Tests testGetAddlParamsString and testGetAddlParamsStringWithHashedEmail failing

• Caused by the expected params string to have the wrong ordering to the addlParams. This is because Swift dictionaries do not have ordering. Therefore, upon creation of the addlParams dictionary, the ordering of the addlParams was changed.
• Fixed by changing testing strategy to check whether each param is contained inside the generated string in different assertion statements.

[fsargent]

This is great. I think you should be able to get the tests to pass by upgrading the .travis.yml with xcode10.2
https://docs.travis-ci.com/user/reference/osx/#xcode-version

[FodT]

Thanks for this PR!
Right now, the travis build is failing. Can you update .travis.yml to use xcode10.2 ?

[mtjoe]

Travic CI is passing now 👍

[FodT]

Thank you!