Update travis and dependencies

[Hkly]

• Newer version of babel requires use of babel preset to compile es6 to es5
• Pin superagent at 1.2.0 to avoid breaking changes

[snitch]

:sparkles: No lint errors found. :sparkles:

[Hkly]

Closing in favor of https://github.com/yola/pixabayjs/pull/37 which also includes updates to superagent usage