Support Node 10

[mikedidomizio]

With the release of Node 10, the builds started failing here:
https://travis-ci.org/boxing/boxrec/builds/371282493

I noticed that the Node version was 10 and the last successful build was with Node 8. Specifying Node 8 resulted in a successful build:
https://travis-ci.org/boxing/boxrec/builds/371289454

TypeError: Cannot assign to read only property 'asyncIterator' of function 'function Symbol() { [native code] }'

This line may have something to do with it:
https://github.com/boxing/boxrec/blob/master/src/boxrec.class.ts#L9

• The fix should also support Node 8
• Possibly should be a fix release for this

[mikedidomizio]

Fixed the issue

Merge Request: #48
Build with Node 10 enabled: https://travis-ci.org/boxing/boxrec/builds/381938578

There are some warnings in the build, seems to be related to this

Kept .travis.yml on Node 8 for the time being