Switch to ESLint for formatting/linting

[eriwen]

JSCS has joined with ESLint, so we should switch to that for linting.

Expected Behavior

• Update package.json
• Migrate the current settings in .jscsrc to .eslintrc with equivalent rules.
• Update gulp config to use eslint instead
• Update travis CI config to run eslint

[eriwen]

This is done.