Update dev dependencies

[fanatid]

Not a big deal, but wanted keep at actual stage for development.

• istanbul removed, if coverage will be required will need add scripts and add nyc as dependency
• semistandard updated and new error messages fixes, probably new version will out soon (new standard version was released 2 days ago)
• mocha updated, nothing changed

Added yarn.lock for locking dev dependencies, travis now install dependencies with yarn.

Node.js versions in travis config updated, it's not possible keep 7,9 because one of dependencies of dependencies have restrictions in engines field, details: https://travis-ci.org/fanatid/bn.js/builds/555443389 Version 6 is not possible, because assert package at that time did not had strictversion (require('assert').strict). I think it's possible to solve by using strictEqual and other, but is this matter run tests on version 6?

Going squash and merge after 24h if there will no NACKs.

[fanatid]

Last commit need to be fixed a little, fix in eslint-plugin-es5: https://github.com/nkt/eslint-plugin-es5/pull/34