Handle cases when vipAddress may not be present

[mhamann]

Fixes #101.

[awolden]

@mhamann Thanks for catching this. We can merge this and publish the fix immediately, but it looks like the build failed on a lint error:

/home/travis/build/jquatier/eureka-js-client/src/EurekaClient.js
  407:1  error  Trailing spaces not allowed  no-trailing-spaces
✖ 1 problem (1 error, 0 warnings)

If you can fix the trailing space and push we can merge this ASAP.

[mhamann]

Fixed linting check and added another test for the case in question to avoid reducing code coverage.

[awolden]

Looks good 👍