Add megacheck to TravisCI checks

[mbernier]

Issue Summary

Issue #215, came to be due to a code checking tool called Megacheck

It would be awesome to have this running and breaking Travis builds if it finds something wrong with this library.

[mbernier]

I created a similar Issue in our 2 other Go repos:

• Add Megacheck to TravisCI build for smtpapi-go smtpapi-go#50
• Add megacheck to sendgrid/rest TravisCI checks rest#57

[vaskoz]

I'll take this one.

[vaskoz]

Why use a single linter like megacheck when you can use all-of-them with GoMetaLinter?