chore(package.json): upgrade gulp-jshint to v1.5.3

This version of gulp-jshint depends on jshint@2.5.0, which includes some basic
support for ES6 template literals.

Closes #26
Closes #27
style(watchtower): use template literals rather than explicit string operations

Template literals are nice to look at, and jshint won't yell at us for using them,
anymore.